Dihybrid Crosses - looking at 2 traits

Mendel’s dihybrid crosses: 1.Mendel also performed crosses involving two pairs of traits, e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green). 2.If alleles sort independently, four possible phenotypes (2 n ) appear in the F 2 generation in a 9:3:3:1 ratio. “Mendel’s Principle of Independent Assortment”: Alleles for different traits assort independently of one another. Modern formulation of independent assortment: Genes on different chromosomes behave independently in gamete production.

The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment The alignment of one pair of homologs is independent of any other. Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. RREE RE All gametes are RE

Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. RREe ReRE Re You have 2 possible gametes: Re and RE

Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. rrEe rerE re You have 2 possible gametes: rE and re

How many gametes? To determine the number of different gametes a parent can have use the FOIL method. TTYY = Number and kind of gametes one TY TtYY TtYy = = twoTY and tY four Ty, tY, ty, TY

If I cross: What are the different possible gametes for these parents?? RRTT x RrTt Possible Gametes (sperm/egg): RTRT, Rt, rT, rt There are 4 total different gametes

If I cross: What are the possible gametes? RRTT x RrTt RT rtrTRtRT RRTTRrTtRrTTRRTt I have 4 possible offspring

If I cross: What are the possible gametes and offspring? rrTt x RRTt rT RtRT RrTTRrTt I have 4 possible offspring rt RrTtRrtt

If I cross: What are the possible gametes? RrTt x RrTt RT rtrTRtRT RRTTRrTtRrTTRRTt I have 16 possible offspring 9:3:3:1 Rt rT rt RRTt RRttRrTtRrtt RrTTRrTtrrTTrrTt RrTtRrttrrTtrrtt

Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another? Because it’s the only way to explain the pattern of inheritance. Phenotype Ratio: 9 yellow/smooth 3 yellow/wrinkled 3 green/smooth 1 green/wrinkled

Consider a cross between parents heterozygous for both deafness and albinism. This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape. What Works for Peas Also Works for Humans

Exceptions to the rule…

Some alleles are related through - Incomplete Dominance Dominance relationships may differ, but the Law of Segregation remains the same.

Multiple Alleles Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. The human ABO blood group is determined by three alleles (I A, I B, and i) of a single gene. The AB phenotype (genotype I A I B ) is an example of codominance

Codominance The human ABO blood group illustrates another genetic phenomenon – codominance. Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote. Neither allele is dominant or recessive

Incomplete or Codominant? +

Pleiotropy – When One Allele Influences Many Traits Examples: 1.Sickle Cell Anemia 2.Albinism

Pleiotropy in Action Anemia, infections, weakness, impaired growth, liver and spleen failure, death. Traits (phenotypes) associated with the sickle cell allele.

Polygenic Inheritance – when a Single Trait is influenced by many genes Examples: 1.Height 2. Hair texture 3. Skin color 4. eye color