Mendelian Genetics

Question 1

Alleles

Answer 1

Alternative versions of a gene found at the same locus in homologous chromosomes

Question 2

Law of Segregation

Answer 2

the two alleles for a heritable character separate and segregate during gamete production and end up in different
gametes.
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis.
 If an organism has two identical alleles for a particular character, then that allele is present as a single copy in all gametes.
 If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other.
 Mendel’s law of segregation accounts for the 3:1 ratio that he observed in the F2 generation

Question 3

Mendel’s law of independent assortment

Answer 3

each pair of alleles segregates independently during gamete formation.
 Strictly speaking, this law applies only to genes located on different, nonhomologous chromosomes.
 Genes located near each other on the same chromosome tend to be inherited together and have more complex inheritance patterns than those predicted for the law of independent assortment

Question 4

Concept 14.3: Inheritance patterns are often more
complex than predicted by simple Mendelian genetics

Answer 4

• The relationship between genotype and
  phenotype is rarely as simple as in the pea plant
  characters Mendel studied
• Many heritable characters are not determined by
  only one gene with two alleles
• However, the basic principles of segregation and
  independent assortment apply even to more
  complex patterns of inheritance

Question 5

Extending Mendelian Genetics for a Single Gene

Answer 5

• Inheritance of characters by a single gene may
  deviate from simple Mendelian patterns in the
  following situations:
  – When alleles are not completely dominant or
  recessive
  – When a gene has more than two alleles
  – When a gene produces multiple phenotypes

Question 6

Complete dominance

Answer 6

when phenotypes of the heterozygote and dominant homozygote are identical

Question 7

In codominance

Answer 7

two dominant alleles affect the phenotype in separate, distinguishable ways

Question 8

In incomplete dominance

Answer 8

The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Question 9

Frequency of Dominant Alleles

Answer 9

• Dominant alleles are not necessarily more
  common in populations than recessive alleles
• For example, one baby out of 400 in the United
  States is born with extra fingers or toes
• The allele for this unusual trait is dominant to the
  allele for the more common trait of five digits per
  appendage
• In this example, the recessive allele is far more
  prevalent than the dominant allele in the
  population

Question 10

The Relation Between Dominance and Phenotype

Answer 10

• A dominant allele does not subdue a recessive
  allele; alleles don’t interact
• Alleles are simply variations in a gene’s nucleotide
  sequence
• For any character, dominance/recessiveness
  relationships of alleles depend on the level at
  which we examine the phenotype

Question 11

Multiple Alleles

Answer 11

• Most genes exist in populations in more than two
  allelic forms
• For example, the four phenotypes of the ABO
  blood groups

Question 12

Pleiotropy

Answer 12

• Most genes have multiple phenotypic effects, a
  property called pleiotropy
• For example, pleiotropic alleles are responsible for
  the multiple symptoms of certain hereditary
  diseases, such as cystic fibrosis and sickle-cell
  disease

Question 13

Extending Mendelian Genetics for Two or More Genes

Answer 13

Some traits may be determined by two or more genes

Question 14

Epistasis

Answer 14

• In epistasis, a gene at one locus alters the
  phenotypic expression of a gene at a second
  locus
• For example, in mice and many other mammals,
  coat colour depends on two genes
• One gene determines the pigment colour (with
  alleles B for black and b for brown)
• The other gene (with alleles C for pigment colour
  and c for no pigment colour ) determines whether
  the pigment will be deposited in the hair

Question 15

Polygenic Inheritance

Answer 15

Quantitative characters are those that vary in the
population along a continuum
* Quantitative variation usually indicates polygenic
inheritance, an additive effect of two or more
genes on a single phenotype
* Skin color in humans is an example of polygenic
inheritance

Question 16

Nature and Nurture: The Environmental Impact
on Phenotype

Answer 16

• Another departure from Mendelian genetics
  arises when the phenotype for a character
  depends on environment as well as genotype
• The norm of reaction is the phenotypic range of
  a genotype influenced by the environment
• For example, hydrangea flowers of the same
  genotype range from blue-violet to pink,
  depending on soil acidity

Question 17

Integrating a Mendelian View of Heredity and
Variation

Answer 17

An organism’s phenotype includes its physical
appearance, internal anatomy, physiology, and
behavior
* An organism’s phenotype reflects its overall
genotype and unique environmental history

Question 18

Pedigree Analysis

Answer 18

A pedigree is a family tree that describes the
interrelationships of parents and children across
generations
* Inheritance patterns of particular traits can be
traced and described using pedigrees
* Pedigrees can also be used to make predictions
about future offspring
* We can use the multiplication and addition rules to
predict the probability of specific phenotypes

Question 19

Recessively Inherited Disorders

Answer 19

• Many genetic disorders are inherited in a
  recessive manner
• Recessively inherited disorders show up only in
  individuals homozygous for the allele
• Carriers are heterozygous individuals who carry
  the recessive allele but are phenotypically normal

Question 20

Cystic Fibrosis

Answer 20

• Cystic fibrosis is the most common lethal genetic
  disease among people of European descent,
  striking one out of every 2,500
• The cystic fibrosis allele results in defective or
  absent chloride transport channels in plasma
  membranes
• Symptoms include mucus buildup in some internal
  organs and abnormal absorption of nutrients in the
  small intestine

Question 21

Sickle-Cell Disease

Answer 21

• Sickle-cell disease affects one out of 400 AfricanAmericans
• The disease is caused by the substitution of a
  the single amino acid in the haemoglobin protein in
  red blood cells
• Symptoms include physical weakness, pain, organ
  damage, and even paralysis
• About 1 in 10 African Americans are carriers

Question 22

Whatis a gene?

Answer 22

A gene is a sequence of nucleotides which forms part of the chromosome which code for specific amino acids in proteins. The gene is the unit of inheritance in organisms, which are passed onto to offspring, dictating features such as their physical appearance and genetic makeup. The famous Scientist, Gregor Mendel, conducted pea plant experiments which framed our initial understanding of inheritance and how traits are passed down to the offspring. Mendel chose plants that had distinct traits, such as seed colour and stem length, to directly observe how traits were passed down from one generation to the next. Mendel repeatedly cross-fertilized pea plants and recorded which traits were expressed and not expressed in the offspring plant from the parent plant. Mendel discovered that genes were hereditary units that displayed predictable patterns of inheritance and followed the principles of independent assortment and segregation. Following Mendel’s theories, Watson and Crick significantly developed knowledge related to genes, specifically the structure of DNA which carries the genetic information of the organism. Genes were discovered to consist of sequences of many nucleotides within the DNA molecules. Genes were now recognised as molecules capable of synthesising proteins based on the order of these DNA nucleotides.