Chromosomes and Heredity

When Gregor Mendel formulated his laws of inheritance of traits, he did not know about meiosis or the existence of chromosomes in cells.

Mendel concluded that each trait has two factors (now known as alleles)

We know now though, that during gamete formation, the factors segregate so that each gamete has one factor.

One factor is donated from our mother, the other from our father.

ChromosomeTheory of Inheritance

states that genes are located on chromosomes, and chromosomes provide the basis for the segregation and independent assortment of genes

Sex-linked Inheritance

A normal person has two sex chromosomes: male (XY) female (XX)

The transmission of genes that are located on one of the sex chromosomes, X or Y, is called sex-linked inheritance

A gene that is located on the X chromosome only is called X-linked. (in humans, include colour blindness and Hemophilia)

A gene that is located on the Y chromosome only is called Y-linked. (hairy pinna)

If you were colourblind, this imagewould appear as a tan or light brown colour.

Sex-linked Inheritance

Most known sex-linked traits are X-linked. Very few Y-linked traits are known.

Y chromosome is much smaller than the X chromosome therefore carries less genes.

Morgan and the Fruit Flies

In 1910, Thomas Morgan was investigating eye colour in fruit flies.

Produced white-eyed male by crossing two red-eyed parents.

(How can this be explained?)

Morgan and the Fruit Flies

When White eyed male X Red-eyed female, he was not able to produce a white-eyed female.

Hypothesized that eye colour located on the X chromosome.

White-eyed trait is recessive.

What would be the genotype of a white-eyed female?

Sex-linked Traits and Punnett Squares

When creating a Punnett square to determine the outcome of a cross involving a sex-linked trait, assume that the trait is located on the X chromosome unless it is stated that the trait is located on the Y chromosome.

Treat p.square in the same manner as autosomal traits. The only differences are that the alleles are written on the X chromosome if the trait is X-linked and no allele is written with the Y chromosome.

When the genotype and phenotype ratios are written, the sex is always included in the ratio

Chromosomesand Gene Expression

Males and females produce the same amounts of proteins coded by genes located on the X chromosome.

This is because one of the female X’s are inactivated in each cell.

Barr body: the inactive X chromosome

Inactivated X chromosome The tortoiseshell coat colour in

cats is an example of the presence of inactivated X chromosomes.

The tortoiseshell pattern can only occur in female cats

Each tortoiseshell cat has a random distribution of orange and black patches.

The gene that codes for this orange and black colour is located on the X chromosome.

A tortoiseshell cat is heterozygous for the allele that codes for this coat colour.

This means that one copy of the X chromosome has an allele that codes for orange pigment and the other copy of the X chromosome has an allele that codes for black pigment.

The random inactivation of an X chromosome in every cell of a tortoiseshell cat results in a patchwork orange and black coat colour.

Polygenic Inheritance

Many traits are controlled by more than one gene known as polygenic inheritance.

protein products produced by many genes working together results in a range of variation, or continuous variation - a gradient of phenotypes for one trait in a population

Ex// ?

Ear length

In humans, height and skin colour are thought to be polygenic traits.

AABB people

Chapter Review

Chapter 4 & 5 Quiz on Tuesday! Review: page 147

(list is in the ‘other patterns of inheritance ppt.)

Review: page 181 #1-7, 10-13, 15-20, 21-23, 26-28, 31-32,

34, 39-40.