Lecture XI: Mendelian Genetics and Beyond

Question 1

The Law of Independent Assortment:

Answer 1

Alleles from different heritable characters separate independently of each other during gamete formation (meiosis).
During Metaphase I of Meiosis:

Maternal and paternal copies of chromosomes (homologous pairs) align side by side on the midline of the cell.

The side of the midline that each ends up on is random and completely independent from the alignment of other pairs of homologous chromosomes.

Question 2

If the parent’s genotype is TTRR, what combinations of alleles could the gametes have? Keep in mind that gametes will have only one copy (allele) of each gene. (assort independently)

Answer 2

TR

Question 3

But, if the parent’s genotype is TtRr, the possible combinations of alleles in the gametes will be:
(assort independently)

Answer 3

TR , Tr , tR , tr

Question 4

Mendel’s Experiment:

Answer 4

Tested to see whether 2 traits are inherited independently of each other.

Question 5

Di-Hybrid Cross

Answer 5

When individuals are crossed that are heterozygous for 2 independent characteristics
YyRr x YyRr

Question 6

Independent vs Dependent assortment

Answer 6

Let genotype be YyRr:
if dependent: YR and yr
If independent: YR, yr, Yr, yR

Question 7

Genes on the same chromosome

Answer 7

Alleles on the same chromosome are most often inherited together and therefore are considered linked. The closer they are situated together on a chromosome the more likely that they will remain together during crossing over. Genes closer to the centromere are less likely to be exchanged between homologous chromosomes.
(during crossing over)

Question 8

What are the other factors that affect genetics?

Answer 8

Dominant alleles are not always more prevalent than recessive ones.

Degrees of dominance can vary.

Most genes have more than 2 possible alleles.

Pleiotropy (one gene has multiple phenotypic effects)

Epistasis (one gene affects the expression of another gene) (fur color and albinism)

Polygenic Inheritance (multiple genes contribute to the phenotypic expression of a single characteristic)

Gene expression is affected by the environment.

Question 9

1. Dominant alleles for a trait are not always more common than the recessive alleles.

Answer 9

e.g. The allele for polydactyly is dominant over the allele for hands and feet with 5 digits only.

Natural selection helps keep the recessive allele more common in nature.

Question 10

2. Degrees of dominance can vary

Answer 10

Complete dominance: Dominant allele expressed when genotype homozygous dominant or heterozygous.
Like Mendel’s characters.

Incomplete dominance: Neither allele is completely dominant. Heterozygotes have a phenotype intermediate between the 2 homozygous forms.

Codominance: Both alleles are fully expressed in the phenotype of heterozygotes.

Question 11

3. Multiple Alleles for one gene

Answer 11

Blood types: Both Ia and Ib ar codominant, I is recessive to the other two alleles.

Question 12

6.Polygenic Inheritance (multiple genes contribute to the phenotypic expression of a single characteristic)

Answer 12

Pleiotropy one genes affect many phenotypic characters.
Polygenic inhetitance: Many genes affect 1 phenotypic character.

Three genes determine skin pigmentation

Question 13

7. Gene expression is affected by the environment

Answer 13

General Rule: Genes set the limits for the phenotype, and the environment molds the phenotype within its limits
Ex.
Inheritance of height in humans
Genes set limit (ex not 10 feet tall), but environment shapes person within his/her limits
Modified by variety of environmental conditions: diet, general health, sleep, etc.