Non-Mendelian Genetics i.e. exceptions to Mendel’s Rules.

Non-Mendelian Genetics

• i.e. exceptions to Mendel’s Rules

Recall Incomplete Dominance

•pattern of gene expression in which the phenotype of a heterozygous individual is intermediate between those of the parents.

Codominance

• the condition in which both alleles in a heterozygous organism are expressed.

• Ex. Roan horses or cows

Multiple Alleles

• genes with three or more alleles

• Ex. Blood types- there is an A, B, and O allele (IA, IB, i)

• (More on this when we discuss blood type genetics)

Polygenic Trait-• when several genes influence one trait.• Ex. Eye color, height, hair, and skin color

Height is a polygenic trait

Skin color is a polygenic trait. The greater the total number of dominant alleles, the darker skin color an individual will have. For this example, assume 3 genes control this trait as shown in the chart:

6 dominant (ex. AABBCC) – extremely dark skin5 dominant (ex. AABBCc)– very dark skin4 dominant (ex. AaBBCc)- dark skin3 dominant (ex. AaBbCc) - olive skin2 dominant (ex. aaBbCc) - light skin1 dominant (ex. aaBbcc)- very light skin0 dominant (ex. aabbcc) – extremely pale skin

If a father with the genotype AABbcc has a child with a mother whose genotype is aabbCc, what are the chances their child will have the genotype AaBbCc? What would the phenotype of this child be?

Pleiotropy

• one gene that affects more than one seemingly unrelated phenotypes

Anemia, infections, weakness, impaired growth, liver and spleen failure, death.

Traits (phenotypes) associated with the sickle cell allele.

Epistasis

• one gene masks the expression of a different gene for a different trait

• Ex. The gene for albinism masks the effects of genes for skin, hair, and eye color.

Influence of Environment

• phenotype depends on conditions in the environment.

• Ex. Arctic fox, Siamese cats, height, and skin color

Temperature sensitive – The cold extremities (ears, nose, tail, and feet) express pigmentation while the warm body does not)

Practice Problems

INCOMPLETE DOMINANCE:

• Yellow coat color in guinea pigs is produced by the homozygous genotype YY

• Cream color by the heterozygous genotype Yy

• White by the homozygous genotype yy.

What genotypic and phenotypic ratios are matings between cream-colored individuals likely to produce?

1:2:1yellow: cream: white

CO-DOMINANCE:• In cattle, reddish coat color is

not completely dominant to white coat color. Heterozygous individuals have coats that are roan colored (ie. reddish, but with spots of white hairs).

• What would be the results of the following crosses:

a. red x roan:b. white x roan:c. red x white:d. roan x roan:

Genotype phenotype 1:1 RR:Rr 1:1 Red: Roan

1:1 Rr:rr 1:1 Roan:WhiteAll Rr all Roan

1:2:1 RR:Rr:rr 1:2:1 Red:Roan:White

• Outline a breeding procedure whereby a pure breeding strain of red cattle could be established from a roan bull and a white cow:– First cross a roan bull with a white cow; next cross

two roan progeny and 25% should be red.

Height in humans is polygenic. Each upper case letter adds 3”. Males (aabbcc) are 5’ and females (aabbcc) are 4’ 7”.

How tall would a male with the genotype AaBbCc be?5’9”

A female? 5’4”

Probability that mating with this male & female result in a child that is homozygous recessive for all three traits?(1/4)3 = 1/64

How tall would that child be if it were a girl?4’7”

Epistasis• In Labs, black is dominant

to chocolate (B or b). Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e.

• Determine the number of chocolate labs produced from a black female and a yellow male (BbEe x bbee)

Phenotype Possible Genotypes

BBEE BbEE BBEe BbEe

bbEEbbEe

BBeeBbeebbee

BbEe x bbeeBE Be bE be

be BbEe Bbee bbEe bbee

Color Black Yellow Brown Yellow

Pea Plants• In sweet peas, purple flower color (P) is dominant over white (p),

but there is also a control gene such that if the plant has a “C”, the purple has “permission” to express itself. – If the plant is “cc”, the purple does not “have permission” to

express itself and the flower will be white anyway. – If a plant with homozygous purple, controlled flowers(CC) is

crossed with a plant with white, non-controlled(cc) flowers, diagram the Punnett square for the F1 and F2 generations and calculate the phenotype ratios.• First, what are the genotypes of the parents in the first generation?

PPCC and ppcc• What are the genotypes of their offspring?

PpCc

• What is always the phenotypic ratio for a dihybrid cross:

genotype

P-C-P-ccppC-ppcc

9: Dominant for both traits

3: Dominant for first trait and recessive for second

3: Recessive for first train and dominant for second

1: Recessive for both traits

What are the phenotypes?P-C- purpleP-cc whiteppC- whitePpcc white