Chapter 2 Flashcards
(14 cards)
The Biology of Heredity
Each egg and sperm cell contains 23 chromosomes, tiny structures
in the nucleus that contain genetic material
In 1978, Louise Brown captured the world’s attention as the first test-tube baby conceived in a laboratory dish instead of in her mother’s body. Today, assisted reproductive technology is no longer experimental; it is used more than 130,000 times annually with American women, producing more than 50,000 babies
The best known, in vitro fertilization, involves mixing sperm and egg together in a laboratory dish and then placing several fertilized eggs in a woman’s uterus.
- Only about one-third of the attempts at in vitro fertilization succeed. What’s more, when a woman becomes pregnant, she is more likely to have twins or triplets because multiple eggs are transferred to increase the odds that at least one fertilized egg will implant in the mother’s uterus.
- She is also at greater risk for giving birth to a baby with low birth weight or birth defects.
The first 22 pairs of chromosomes are called autosomes; the chromosomes in each pair are about the same size. In the 23rd pair, however, the chromosome labeled X is much larger than the chromosome labeled Y. The 23rd pair determines the sex of the child; hence, these two are known as the sex chromosomes.
Each chromosome actually consists of one molecule of deoxyribonucleic acid-DNA for short. The DNA molecule resembles a spiral staircase.
Each group of nucleotide bases that provides a specific set of biochemical instructions is a gene.
A child’s 46 chromosomes include about 30,000 genes (Pennisi, 2005). Chromosome 1 has the most genes (nearly 3,000) and the Y chromosome has the fewest (just over 200).
fewer than 1% of genes cause differences between people
The complete set of genes makes up a person’s heredity and is known as the person’s genotype. Through biochemical instructions that are coded in DNA, genes regulate the development of all human characteristics and abilities. Genetic instructions, in conjunction with environmental influences, produce a phenotype, an individual’s physical, behavioral, and psychological features.
Single Gene Inheritance
Genes come in different forms that are known as alleles.
Sometimes the alleles in a pair of chromosomes are the same, which makes them homozygous. Sometimes the alleles differ, which makes them heterozygous.
How does a genotype produce a phenotype?
- The answer is simple when a person is homozygous. When both alleles are the same and therefore have chemical instructions for the same phenotype, that phenotype results.
- When a person is heterozygous, the process is more complex. Often one allele is dominant, which means that its chemical instructions are followed whereas instructions of the other, the recessive allele, are ignored.
Sometimes one allele does not dominate another completely, a situation known as incomplete dominance. In incomplete dominance, the phenotype that results often falls between the phenotype associated with either allele.
Individuals with one dominant and one recessive allele have sickle-cell trait: In most situations they have no problems, but when they are seriously short of oxygen they suffer a temporary, relatively mild form of the disease.
Cultural Influences
Sickle-cell disease affects about 1 in 400 African American children.
Surprisingly, because the sickle-cell allele has a benefit: Individuals with this allele are more resistant to malaria, an infectious disease that is one of the leading causes of childhood death worldwide. Malaria is transmitted by mosquitoes, so it is most common in warm climates, including many parts of Africa. Compared to Africans who have alleles for normal blood cells, Africans with the sickle-cell allele are less likely to die from malaria, which means that the sickle-cell allele is passed along to the next generation.
First, sickle-cell disease should be found in any group of people living where malaria is common.
Second, malaria is rare in the United States, which means that the sickle-cell allele has no survival value to African Americans.
INHERITED DISORDERS
Relatively few serious disorders are caused by dominant alleles
- An exception is Huntington’s disease, a fatal disease characterized by progressive degeneration of the nervous system. Huntington’s disease is caused by a dominant allele found on chromosome 4. Individuals who inherit this disorder develop normally through childhood, adolescence, and young adulthood. However, during middle age, nerve cells begin to deteriorate, causing muscle spasms, depression, and significant changes in personality.
Fortunately, most inherited disorders are rare. PKU, for example, occurs once in every 10,000 births, and Huntington’s disease occurs even less frequently.
Albinism
- 1 in 15,000 births
- Skin lacks melanin, which causes visual problems and extreme sensitivity to light.
Cystic fibrosis
- 1 in 3,000 births among European Americans; less common in African and Asian Americans
- Excess mucus clogs respiratory and digestive tracts. Lung infections common.
Phenylketonuria (PKU)
- 1 in 10,000 births
- Phenylalanine, an amino acid, accumulates in the body and damages the nervous system, causing mental retardation.
Tay-Sachs disease
- 1 in 2,500 births among Jews of European descent
- The nervous system degenerates in infancy, causing deafness, blindness, mental retardation, and, during the preschool years, death.
ABNORMAL NUMBER OF CHROMOSOMES.
Extra or missing autosomes always
affect development because autosomes contain so much genetic material.
- In fact, nearly half of all fertilized eggs abort spontaneously within 2 weeks, primarily because of abnormal autosomes. Thus, most eggs that could not develop normally are removed naturally
Sometimes individuals do not receive the normal complement of 46 chromosomes. If they are born with extra, missing, or damaged chromosomes, development is always disturbed.
The best example is Down syndrome, a genetic disorder that is caused by an extra 21st chromosome and that results in mental retardation.
- persons with Down syndrome have almond-shaped eyes and a fold over the eyelid. The head, neck, and nose of a child with this disorder are usually smaller than normal.
- During the first several months, babies with Down syndrome seem to develop normally. Thereafter, though, their mental and behavioral development begins to lag behind the average child’s.
- life expectancy ranges from 25 to 60 years
What causes Down syndrome?
- Individuals with Down syndrome typically have an extra 21st chromosome that is usually provided by the egg
Klinefelter’s syndrome
- XXY
- 1 in 500 to 1,000 male births
- Tall, small testicles, sterile, below-normal intelligence, passive
XVY complement
- XYY
- 1 in 1,000 male births
- Tall, some cases apparently have below- normal intelligence
Turner’s syndrome
- X
- 1 in 2,500 to 5,000 female births
- Short, limited development of secondary sex characteristics, problems perceiving spatial relations
XXX syndrome
- XXX
- 1 in 500 to 1,200 female births
- Normal stature but delayed motor and language development
Behavioral Genetics
Behavioral genetics is the branch of genetics that deals with inheritance of behav- ioral and psychological traits.
The traits controlled by single genes represent “either-or” phenotypes. That is, the genotypes are usually associated with two (or sometimes three) well-defined phenotypes.
Most important behavioral and psychological characteristics are not either-or cases but represent an entire range of different outcomes. Like a distribution gradient
- Many behavioral and psychological characteristics, including intelligence and aspects of personality, are distributed in this fashion, with a few individuals at the ends of the continuum and most near
the middle.
- Phenotypes distributed like this often reflect the combined activity of many separate genes, a pattern known as polygenic inheritance.
METHODS OF BEHAVIORAL GENETICS
Identical twins are called monozygotic twins because they come from a single fertilized egg that splits in two.
In contrast, fraternal or
dizygotic twins come from two separate eggs fertilized by two separate sperm.
Adopted children are another important source of information about heredity.
Twin studies and adoption studies are powerful tools. They are not foolproof, however.
- Maybe you thought of a potential flaw in twin studies: Parents and other people may treat identical twins more similarly than they treat fraternal twins. This would make identical twins more similar than fraternal twins in their experiences as well as in their genes.
- Adoption studies have their own Achilles’ heel. Adoption agencies some- times try to place youngsters in homes like those of their biological parents.
Twin study
- Compares monozygotic and dizygotic twins
- Monozygotic twins more alike than dizygotic twins
- Others may treat monozygotic twins more similarly than they treat dizygotic twins
Adoption study
- Compares children with their biological and adoptive parents
- Children more like biological parents than adoptive parents
- Selective placement: Children’s adoptive parents may resemble their biological parents
Today, researchers are able to isolate particular segments of DNA in human chromosomes.
- These segments then serve as markers for identifying specific alleles.
- The procedure is complicated, but the basic approach often begins by identifying people who differ in the behavior or psychological trait of interest.
WHICH PSYCHOLOGICAL CHARACTERISTICS ARE AFFECTED BY HEREDITY?
Research reveals consistent genetic influence in many psychological areas, including personality, mental ability, psychological disorders, and attitudes and interests.
There is impact of heredity on cognitive processes and reading skill.
- Vocabulary scores were correlated .59 for identical twins but .36 for fraternal twins (Kovas et aI., 2005).
- Scores on a measure of sex-typed play-that is, the extent to which boys like masculine sex-typed toys and activities while girls like feminine sex-typed toys and activities-were correlated .83 for male identical twins but .69 for male fraternal twins and .78 for female identical twins but .50 for female fraternal twins (Iervolino et aI., 2005).
- Scores on a measure of aggressive play with peers were correlated .64 for male identical twins but .34 for male fraternal twins and .54 for female identical twins but .36 for female fraternal twins (Van Hulle, Lemery-Chalfant, & Gold- smith, 2007).
the impact of heredity on behavioral development is substantial and widespread. Heredity has a sizable influence on such different aspects of development as intelligence and personality.
On the other hand, heredity is never the sole determinant of behavioral development. If genes alone were responsible, then identical twins should have identical behavioral and psychological phenotypes.
Paths from Genes to Behavior
- The consequences of genetic instructions depend on the environment in which those instructions develop.
- In other words, a genotype can lead to many different phenotypes, depending on the specific environment in which the genotype is expressed
- Reaction range refers to the fact that the same genotype can produce a range of phenotypes, in reaction to the environment where development takes place.
- In fact, behavioral geneticists often use correlations from twin and adoption studies to calculate a heritability coefficient, which estimates the extent to which differences between people reflect heredity.
- Why be cautious? One reason is that many people mistakenly interpret heritability coefficients to mean that 50% of an individual’s intelligence is due to heredity; this is incorrect because heritability coefficients apply to groups of people, not to a single person. A second reason for caution is that heritability coefficients apply only to a specific group of people living in a specific environment. - Heredity and environment interact dynamically throughout development.
- In fact, genes and environments constantly influence each other throughout a child’s life
- First, genes are expressed- “turned on”-throughout a person’s development.
- Second, the environment can trigger genetic expression: Children’s experiences can help to determine how and when genes are activated - Genes can influence the kind of environment to which a child is exposed.
- In other words, “nature” can help determine the kind of “nurturing” that a child receives
- A child’s genotype can lead people to respond to the child in a specific way.
- For example, imagine a child who is bright
and outgoing (both due, in part, to the child’s genes). That child may receive plenty of attention and encouragement from teachers.
- In addition, as children grow and are more independent, they actively seek environments related to their genetic makeup. Children who are bright (due in part to heredity) may actively seek peers, adults, and activities that
strengthen their intellectual development.
- This process of deliberately seeking environments that fit one’s heredity is called niche-picking. Niche-picking is first seen in childhood and becomes more common as children get older and can control their environments.
- Niche-picking is a prime example of the interaction between nature, nurture, and development. Experiences determine which phenotypes emerge, and genotypes influence the nature of children’s experiences. - Environmental influences typically make children within a family different.
- Children within a family should be similar because they all receive the same type of effective (or ineffective) parenting. However, dozens of behavioral genetic studies show that, in reality, siblings are not very much alike in their cognitive and social development
- These findings point to the importance of nonshared environmental influences, the environmental forces that make siblings different from one another
Principles of Genetics
For single-gene traits, phenotype expressed through combinations of dominant and recessive alleles
Allele = one variant of the gene
The term allele refers to the specific form of a gene. Recall that you get one of each chromosome from your father and one from your mother and thus have one pair of each gene. For some genes, one member of the pair is dominant, in that it always gets expressed regardless of what the other allele is. Nondominant or recessive alleles only show up in the phenotype when both members of a pair are recessive.
Note: most traits are multiply determined (polygenic)
E.g., intelligence, personality
The majority of meaningful human psychological traits (intelligence and personality characteristics), however, are not governed by single gene pairs. People aren’t either exceedingly smart or dull, or either outgoing or shy. Instead, people can fall anywhere along a wide spectrum for these traits, because they’re polygenic, i.e., multiply determined.
Heritability
Def: an estimate of the proportion of trait variability in a population that is determined by genetic differences
E.g., heritability of height = 90%
Note: Accounts for differences within groups, not within individuals
If the heritability of height is 90%, that doesn’t mean that each individual’s height is 90% determined by genes, 10% by their environment. Rather, it means that if you’re comparing people within a certain group, that about 90% of the differences in height between people in the group is due to genetic factors, and only 10% to environmental factors. Asking which type of factor has more influence is like asking whether the height or the width of a rectangle has a greater influence on its shape. They are inextricably intertwined, as are genetic factors and environmental factors in shaping us!
Ways of Determining Heritability
- Twin studies
- Adoption studies
The two main sources available to researchers who study the heritability of traits are studies of twins, and of individuals who have been adopted close to birth. With studies of twins, a researcher might compare sets of identical twins raised in the same family with fraternal twins raised in the same family. Rare cases in which identical twins have been adopted by different families at birth and raised apart (and remained unaware of their twin’s existence until adulthood) also offer valuable insights, although in some cases researchers have acted unethically in conjunction with the adoption agencies. In studies of adopted individuals, researchers seek to determine if adopted children are more similar on certain traits to their adoptive parents and siblings (with whom they share an environment) or their birth parents and siblings (with whom they share genes).
Twins
Monozygotic (identical):
union of one egg and one sperm that splits in two, soon after conception
Genetic relatedness: 100%
Dizygotic (fraternal):
two different eggs fertilized by two different sperm
Genetic relatedness: 50%
Monozygotic is the formal term for identical twins. Monozygotic twins result from a fertilized egg splitting in two shortly after conception, for reasons unknown. Identical wins have the exact same genotype – their genetic relatedness is 100 per cent. Pictured above are the McGuire twins, Billy and Benny. They were professional wrestlers who held the Guinness World Record for the heaviest set of twins. Billy died at age 32 as the result of a motorcycle accident, while Benny died of heart failure at age 54.
Dizygotic is the official term for fraternal twins, such as the Olsen twins (above). As dizygotic twins are the result of two different eggs fertilized by different sperm, their genetic relatedness is only 50 per cent, the same as any non-twin siblings born to the same parents.
Twin research is based on the notion that if identical twins (whose genes are exactly the same) are more similar than fraternal twins or non-twin siblings (who are similar, but not identical, in genetic inheritance) reared together, heredity must play an important role in the trait being studied. A potential confound of this type of study is that it’s likely that identical twins also share more similar environments than do fraternal twins. Parents and teachers might treat identical twins more similarly than they treat fraternal twins, and identical twins might have more common friends than fraternal twins. It’s hard to disentangle environmental and genetic factors when comparing identical twins raised in the same household to fraternal twins raised together. More reliable data comes from studying identical twins raised apart.
Let’s take a look at a chart which compares correlations of scores on intelligence tests between pairs of individuals (who either share genes, home environments, or both). Recall that 1.00 represents a perfect positive correlation. We see that the IQ scores of identical twins raised apart still have a very strong positive correlation; this tells us that genes definitely play a role in intelligence. If genes had nothing to do with intelligence, we’d expect a correlation much closer to 0 between identical twins raised separately. However, the correlations are less than perfect, even for identical twins, which shows that environment must also influence intelligence.
The next three lines in the chart look at correlations of IQ scores between parents and their children (biological, living together; biological, living apart, and adopted, living together). Again, the correlations tell us that genetic factors and environmental factors are both at play in determining an individual’s intelligence. If genes played a non-significant role, we’d expect the correlation between parents and the children they gave up for adoption to be much lower. And if the environment was unimportant, we’d expect a lower correlation between IQ scores of parents and their adopted children.
In one twin study (Turkheimer et al., 2003), researchers found that of those raised in homes with higher socioeconomic status (SES), approximately 60 per cent of the variance in the children’s IQ scores was due to genetics (heritability). However, for children raised in low SES homes, only about 40 per cent of the variance of those scores was due to heredity, while approximately 60 per cent was due to their environment. In other words, when the environment is enriched, the impact of genes is more visible. When the environment is impoverished, the impact of environment is more visible. As such, enhancing the experiences of children from lower SES families through interventions (increasing parent education, providing high-quality day care) has a positive impact on their development.
Genotypes, Phenotypes, and Human Behavior
Niche Construction:
- Refers to how behaviors, activities, and choices of individuals actively shape and modify the environment in which they live
E.g.: athletic build: involvement in sports
Niche construction refers to the tendency to actively choose environments that complement our heredity. On a free evening, people who are more introverted will probably choose to stay home and read books, or watch movies with a friend. Whereas those who are more extroverted will choose to go to large parties. If you’re taller than the rest of your classmates in Grade 8, there’s a good chance you will have a spot on the school basketball team. Niche construction explains why pairs of identical twins reared apart who meet each other as adults may find, to their surprise, that they have similar jobs, hobbies, and even food preferences!
