Non-Mendelian GeneticsExceptions to Simple Inheritance

Normal Genetics Refresher• Rules -

– Two possible alleles for a gene– One allele is dominant to the other allele

(represented as a uppercase vs lowercase letter)• When an individual has a heterozygous

genotype, the dominant phenotype is shown

– Each parent passes on one allele to the offspring

• Example problem representing normal Mendelian genetics: When two hybrid tall pea plants are bred what is the expected genotypic and phenotypic ratios of the offspring assuming this falls under normal Mendelian rules? Show work.

Non-Mendelian Genetics

• Patterns of inheritance that do not abide by Mendel’s observations. We will study 5 of these patterns- – Incomplete dominance– co-dominance– multiple alleles– sex-linked inheritance– polygenic inheritance.

Incomplete Dominance• There are two alleles for a gene, but neither is

dominant or recessive. When both alleles are present (the heterozygous condition), the phenotype is a blend of the two alleles. This is seen in flower color in snapdragons. In these plants, one homozygous genotype produces red flowers, the other homozygous genotype produces white flowers, but the heterozygous genotype produces pink flowers. – Since neither allele is dominant or recessive, use

two different uppercase letters to represent each allele.

– Sample problem: In snapdragons, the genotype that produces red flowers is RR. The genotype that produces white flowers is WW. Show the cross between a plant with red flowers and a plant with white flowers. What is the genotypic and phenotypic outcome of such a cross?

– Now cross two pink snapdragons. What is the genotypic and phenotypic probability of such a cross?

Co-Dominance• Pattern of inheritance in which both

alleles are shown independently of each other in the heterozygous condition (instead of pink for the heterozygous phenotype, snapdragons would be red and white striped)– Use two different uppercase letters to

represent each allele– Sample Problem: In shorthorn cattle,

hair color is governed by co-dominant alleles. Red alleles are co-dominant with white alleles, and heterozygotes have red and white hairs. They are called roans. Cross one roan with a white cow. What is the genotypic and phenotypic probability of this cross?

Multiple Alleles• Pattern of inheritance in which there are more

than two possible alleles available in a population for a particular gene. Parents still pass on one allele to their offspring and each offspring as two alleles for the gene.

• Example -Hair color in mice is determined by a single gene with a series of alleles, each resulting in different coloration. There are alleles for black, brown, agouti, gray, albino, and others. The twist here is that the same allele can be dominant or recessive depending on context. Allelic series are often written as agouti > black > albino. This means that agouti is dominant to black, and black is dominant to albino. (And agouti is necessarily also dominant to albino.) If the black allele is in the presence of an agouti allele, the mouse will be agouti because black is recessive to agouti. If that same black allele is paired with an albino allele, the mouse will be black since black is dominant to albino.

Blood Type• Follows both a co-dominant

and multiple allele pattern of inheritance.– Three possible alleles in the

population: IA, IB, and Io

– The IA and IB alleles are co-dominant to each other

– The Io allele is recessive to both IA and IB

– Sample Problem: Show the cross between two people with Type AB blood. What is the genotypic and phenotypic outcome of such a cross

Blood Type (Phenotype)

Genotype

Can donate blood to: Can receive blood from:

O

IoIo

A,B,AB and O(universal donor)

O

AB

IAIB

O, AB

A,B,AB and O(universal receiver)

A

IAIA or IAIo

AB, A

O,A

B

IBIB or IAIo

AB,B

O,B

Sex-Linkage• Pattern of inheritance for genes located on the X chromosome. Females have two X

chromosomes, and therefore 2 alleles for each of these genes, but males only have one X chromosome and thus only have one allele for each of these genes. Males inherit their X chromosome from the female parent and thus only inherit alleles from the female parent for genes located on the X chromosome.

• For females, write two uppercase X’s and put their alleles as superscripts above the X’s– Ex) XAXa - showing she is heterozygous for a gene located on the X chromosome

• For males, write one uppercase X and one uppercase Y and put his one allele as a superscript above the X chromosome– Ex) XAY – showing he has inherited the dominant form of the gene for a gene on the X

chromosome• Notice a male cannot be heterozygous for sex-linked traits because he only has one allele

• – Red-green color-blindness, Duchenne’s muscular dystrophy, and hemophilia are disprders caused

by recessive, sex-linked genes. – Males tend to show recessive, sex-linked disorders at a much higher rate than females because

males only have one X chromosome. So the presence of a single recessive allele will result in the recessive phenotype in males. Since females have two X chromosomes, the presence of a single recessive allele will result in the dominant phenotype, though these females are referred to as carriers because they carry the recessive allele.

• Sample problem - Show the cross between a non- hemophiliac man and a woman who is a carrier for hemophilia. What is the probability that they will have a child with hemophilia? What is the probability any female child will have hemophilia? Any male child?

Polygenic Inheritance• Pattern of inheritance when a phenotype is determined by more than one gene. The

genes may be scattered along the same chromosome or located on different chromosomes. The proteins that are coded for by these genes all take part in determining the phenotype.

• Example: human height, weight, body build, hair color, skin color, intelligence• For these phenotypes, there is wide range of variability among individuals• Example: high blood pressure (hypertension) is not the result of a single "blood

pressure" gene with many alleles (a 120/80 allele, a 100/70 allele, a 170/95 allele, etc.) The phenotype is an interaction between a person's weight (one or more obesity genes), cholesterol level (one or more genes controlling metabolism), kidney function (salt transporter genes), smoking (a tendency to addiction), and probably lots of others too. Each of the contributing genes can also have multiple alleles.