Theoretical Genetics

Beyond Mendel

Topics

• Non-Disjunction• Co-Dominance• Polygenic Traits• Pedigree Chart• Sex Linkage• Linkage Group

Non-Disjunction

• Abnormal segregation of homologues during Anaphase I of meiosis

• Tetrads fail to separate• Homologous chromosomes migrate into the

same cell• Results in gametes with extra, or missing,

chromosomes.• Ex. Trisomy 21 – Down Syndrome

Trisomy 21 - Characteristics

ncbi.nlm.nih.gov

www.cdc.gov/Features/dsDownSyndrome/

Co-Dominance

• Also known as incomplete dominance• Two dominant alleles can both be expressed

in an organism’s phenotype• example

– Blood type: A and B are both dominant

oftentimes more than one

gene influences a phenotype

Polygenic Traits

• A phenotype (physical trait) that is determined by more than one gene, often many

• Usually results in phenotypic values that display an incremental range in a bell shaped curve within a population

• Ex. Human height and weight

Bell Shaped Curve – Normal Distribution

Polygenic Traits show Normal Distribution

Pedigree Chart

Sex Linkage

• Traits, or genes, that are located on the X or Y sex chromosome

• Recall: XX = female XY = male• Many sex linked diseases are recessive and

found on the X chromosome• Females can be heterozygous carriers (or

afflicted), males are always afflicted.• XHXh XhY

Sample Question

What is a difference between autosomes and sex chromosomes?

A. Autosomes are not found in gametes but sex chromosomes are.

B. Sex chromosomes are found in animal cells and autosomes are found in plant cells.

C. Autosomes are diploid and sex chromosomes are haploid.D. Sex chromosomes determine gender and autosomes do not.

Sample Question – Sex Linkage

A male and female with normal color vision each have a father who is color blind. They are planning to have children. Predict, showing your working, the possible phenotypes and genotypes of male and female children.

?

K eym alefem a lea ffec ted m a lea ffec ted fem a le

Linkage Group

• Mendel’s Law of Independent Assortment states that alleles of different traits segregate independent from one another.

• But, this is only true if the genes are located on different chromosomes. Why?

Linkage Group

• A linkage group consists of genes for different traits, found on the same chromosome.

• The alleles of the traits do not separate, they must move into the same gamete together during meiosis.

• The exception is if the alleles are separated by crossing over during prophase I.

The closer two genes are located on a chromosome, the less likely the alleles will separate during crossing over

Sample QuestionIn garden peas, the pairs of alleles coding for seed shape and

seed colour are unlinked. The allele for smooth seeds (S) is dominant over the allele for wrinkled seeds (s). The allele for yellow seeds (Y) is dominant over the allele for green seeds (y).

If a plant of genotype Ssyy is crossed with a plant of genotype ssYy, which offspring are recombinants?

A. SsYy and SsyyB. SsYy and ssYyC. SsYy and ssyyD. Ssyy and ssYy

Sample Question

A cross is performed between two organisms with the genotypes AaBb and aabb.What genotypes in the offspring are the result of recombination?

A. Aabb, AaBbB. AaBb, aabbC. aabb, AabbD. Aabb, aaBb

Sample Question

A polygenic character is controlled by two genes each with two alleles. How many different possible genotypes are there for this character?

A. 2B. 4C. 9D. 16

AaBBaaBBAABBAABbAAbbaabbAabbAaBbaaBb

Sample QuestionIn Zea mays, the allele for coloured seed (C) is dominant over the allele for

colourless seed (c). The allele for starchy endosperm (W) is dominant over the allele for waxy endosperm (w). Pure breeding plants with coloured seeds and starchy endosperm were crossed with pure breeding plants with colourless seeds and waxy endosperm.

(a) State the genotype and the phenotype of the F1 individuals produced as a result of this cross.

genotype............................................................................................................... phenotype................................................................................................................

(b) The F1 plants were crossed with plants that had the genotype c c w w. Calculate the expected ratio of phenotypes in the F2 generation, assuming that there is independent assortment. Use the space below to show your working.