Heredity

Question 1

particulate inheritance

(Mendel’s theory of genetics)

Answer 1

inherited characteristics are carried by discrete units (genes)

Question 2

basics of probability

Answer 2

predicts average outcome if sample is large enough

Question 3

law of dominance

(Mendel’s first law)

Answer 3

when 2 organisms, each homozygous (pure) for 2 opposing traits are crossed:
- offspring will be hybrid (carry 2 different alleles)
- only exhibits the dominant trait (hidden trait is the recessive trait)

Question 4

law of segregation

Answer 4

during formation of gametes, 2 traits carried by each parent seperate

Question 5

monohybrid cross

Answer 5

• (Tt x Tt) is cross between 2 organisms that are each hybrid for 1 trait
• phenotype (appearance) ratio from cross is 3 to 1
• genotype (type of genes) ratio is 1:2:1 (25% homozygous dominant, 50% heterozygous, 25% homozygous recessive)

Question 6

backcross or testcross

Answer 6

way to determinte genotype of individiual showing only dominant trait
- involves cross between real organisms
- individual in question (B/?) crossed with homozygous recessive individual (b/b)
- homozygous dominant if all offspring is B/b
- hybrid if half offspring can show recessive trait

Question 7

law of independent assortment

Answer 7

• cross carried out between 2 individuals that are hybrid for 2+ traits that are not on the same chromosome (dihybrid cross)
• during gamete formation, alleles of a gene for 1 trait will segregate independently from alleles of a gene for another trait

genes will not assort independently if genes are linked

Question 8

the dihybrid cross

Answer 8

cross between individuals that are hybrid for 2 different traits
- cross produces 4 types of gametes: TY, Ty, tY, ty
- 16 genotypes, but a few phenotypes (ratio of 9:3:3:1)

Question 9

incomplete dominance

Answer 9

• characterized by blending
• neither trait is dominant so the genes are written in all capital letters

ex: long watermelon (LL) crossed with round watermelon (RR) produces oval watermelonsd (RL)
when 2 RL are crossed, 25% chance for round, 25% chance for long, and 50% for oval

Question 10

codominance

Answer 10

both traits show

ex: 3 blood groups: M, N, MN that are based on 2 distinctmolecules located on surface of red blood cells
- person can be homozygous for 1 type of moleucle (MM/NN), or be hybrid and have both molecules (MN)

Question 11

multiple alleles

Answer 11

when there are more than 2 allelic forms of a gene

many genes in population exist in only 2 allelic forms

Question 12

pleiotropy

Answer 12

ability of single gene to affect organism in many ways

Question 13

epistasis

Answer 13

2 seperate genes control 1 trait, but 1 gene masks expression of other gene

gene that masks expression of other gene is epistatic to the gene

Question 14

polygenic inheritance

Answer 14

characteristics that result from blending of several seperate genes

ex: skin color, hair color, height…

wide variation in genotypes always results in bell-shaped curve in population

Question 15

genomic imprinting

Answer 15

variation in phenotype depending on if trait is from mother or father
- caused by silencing of particular allele by methylation of DNA during gamete formation
- imprint is carried to all body cells + passed down generation by generation
- imprinted genes located on autosomes instead of X chromosome (sex-linked genes)

Question 16

extranuclear genes

Answer 16

genes located in mitochondria and chloroplasts, where DNA is small, circular, and carries small number of genes
- rare + severe diseases linked to extranuclear genes, defects in mitochondrial genes cause weakness in muscles

mitochondrial DNA inherited only from mother, father’s mitochondria does not enter egg

Question 17

genes and environment

Answer 17

environment can alter expression of genes

Question 18

penetrance

Answer 18

proportion/percentage of individuals in gruop with given genotype that shows expected phenotype

ex: ppl with mutant allele for BRCA1 develop breast cancer, but some people with the allele do not develop breast cancer
reason is unknown, but it might be interaction with some other gene/factor from environment

Question 19

linked genes

Answer 19

genes on same chromosome
linked genes tend to be inherited together and do not assort independently (unless separated by crossover event during meiosis/gamete formation)

more genes than chromosomes, 1000s genes linked

humans have 46 chromosomes, so they have 46 linkage groups

Question 20

sex-linkage

Answer 20

46 human chromosomes: 44 (22 pairs) autosomes and 2 sex chromosomes (X and Y)
- traits on X chromosomes sex-linked
- few genes carried on Y chromosome
- females (XX) inherit 2 copies of sex-linked genes
- if sex-linked trait is due to recessive mutation: female will express phenotype if she carries 2 mutated genes (X-X-) and will be a carrier if she only has one (X-X), will express phenotype with 1 mutated genes if mutation is dominant
- males (XY) will express phenotype if the one X is mutated (X-Y), and recessive traits (color blindness, hemophilia, Duchenne muscular dystrophy) are more common, so they are more likely to suffer with sex-linked conditions more often than females

• all daughters of affected fathers are carriers
• sons cannot inherit sex-linked trait from father because son inherits Y chromosome from father
• son has 50% chance of inheriting sex-linked trait from a carrier mother
• uncommon for female to have recessive sex-linked condition, needs to inherit mutant gene from both parentsr

Question 21

crossover

Answer 21

• genes more likely to be seperated if they are farther apart
• see chiasma (physical bridge built around point of exchange)
• results in recombination

Question 22

linkage mapping

Answer 22

1 map unit = distance within which recombination occurs 1% of the time

Question 23

recombination frequencies

Answer 23

(number of recombinants/total number of offspring) x 100

Question 24

pedigree

Answer 24

• family tree that indicates phenotype of 1 trait for every member
• used to determine how trait is inherited
• females = circle, males = square
• carrier state normally not shown, half-shaded if it is
• shaded shape = exhibits trait

Question 25

**X inactivation - The Barr Body**

Answer 25

- in every somatic cell with more than 2 X chromosome, a random X chromsome will be inactivated - inactive choromsome condenses into dark spot of chromatin and is seen in outer edge of nucleus (Barr Body) - genetic mosaic: some cells have X inactivated, some cells have other X inactivaed ## Footnote - calico cats are female bc of this

Question 26

**mutations**

Answer 26

- any changes in the genome/genetic or chromosomal abnormality - occurs randomly, even if radition and chemicals can cause them - 2 types: **gene mutations** or **chromosome mutations**

Question 27

gene mutations

Answer 27

change in DNA sequence

Question 28

chromosome mutation

Answer 28

**deletion** - when fragment lacking a centromere is lost during cell division **inversion** - when chromosomal fragment reattaches to original chromosome but in reverse orientation **translocation** - when fragment of chromosome becomes attached to nonhomologous chromosome **polyploidy** - when cell or organism has extra sets of chromosomes ## Footnote can be seen under a microsope - karyotype shows size, number, shape of choromosomes + reveals presence of abnormalities - karyotype used to scan for chromosomal abnormalities in developing fetuses

Question 29

**nondisjunction**

Answer 29

- error that sometimes occurs during meiosis, when homlogous chromosomes fail to seperate as they should - can result in abnormal number of chromosomes (**aneuploidy**) - chromosome present in triplicate (**trisomy**) - organism wih cells with extra set of chromosomes is **triploid (3n)** - organism with 4n chromosome number is **tertraploid** - strawberries are **octoploid** - organism with extra sets of chromosomes is **polypoid**