Exceptions to Mendelian Genetics & Pedigrees

Mendel’s work led to the Chromosome Theory Genes are located on chromosomes Separation of genes during meiosis account for the inheritance patterns discovered by Mendel

However… Inheritance of traits by a single gene may deviate from simple Mendelian patterns

The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

Several characteristics make fruit flies a convenient organism for genetic studies: They breed at a high rate A generation can be bred every two weeks They have only four pairs of chromosomes They are easy to keep alive They don’t need a lot of space

In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome

Fig. 15-3

EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + + + w w w w + w  Generation F1 All offspring had red eyes Generation RESULTS F2 Generation CONCLUSION P + X w X w Generation X  Y + w w Sperm Eggs F1 + + + w w Generation w w + w Sperm Eggs + + w w F2 + w Generation + w w w w + w

Morgan discovered sex-linked genes Genes found on the sex chromosomes Usually it’s found on the X chromosome

Difference in Human X and Y Chromosomes

Females are XX, and males are XY Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome

For example, a gene for making a blood clotting protein is found on the X chromosome Having a mutation of this gene causes hemophilia Males that inherit a mutation of the gene found on their X chromosome, lack another gene on their Y chromosome and have hemophilia

Swelling of the joints in hemophiliac patient

Duchenne (pseudohypertropic) muscular dystrophy is also sex-linked

Classic signs of Duchenne MD

8% of males have some type of color blindness 0.4% of females are color blind The most common type is distinguishing red from green

Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical For example: Aa genotype gives the dominant phenotype and the “a” gene is completely hidden

There are exceptions! In incomplete dominance, the phenotype of heterozygotes is somewhere between the phenotypes of the two parental varieties NEITHER the dominant or recessive trait shows up – there’s a third phenotype

Such as inheritance of petal color in Japanese 4 o’clocks Fig. 14-10-3 P Generation Red White CRCR CWCW Such as inheritance of petal color in Japanese 4 o’clocks Gametes CR CW Pink F1 Generation CRCW Gametes 1/2 CR 1/2 CW Sperm 1/2 CR 1/2 CW F2 Generation 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

The sickle cell gene is also incompletely dominant. A person heterozygous has sickle cell trait

In codominance, if two different alleles are present, they will both show their trait For example, ABO human blood types

Sometimes there are more than 2 alleles for a trait and are called multiple alleles Example: the A, B, and O genes in human blood types

Polygenic traits are controlled by more than one pair of genes and accounts for a wide variety of a trait Skin color in humans is an example of polygenic inheritance

Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genes along a particular chromosome He did this through studying how frequently two genes located on the same chromosome separate during crossing over

Sturtevant discovered that some genes are found on the same chromosome very close together and, if present, are often inherited together For example, the genes for hair color and presence of freckles are both found on the same chromosome close to each other and are often inherited together

This deviates from Mendel’s principle of independent assortment

Even though there are exceptions to Mendel’s principles, basic Mendelian genetics endures as the foundation of genetics

A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees

Key Male Affected male Mating Offspring, in birth order Fig. 14-15a Key Male Affected male Mating Offspring, in birth order (first-born on left) Female Affected female

(a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?

(b) Is an attached earlobe a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?