The Chromosomal Basis of Inheritance

Chapter 15

The importance of chromosomes

• In 1902, the chromosomal theory of inheritance began to take form, stating: genes have specific locations (loci) on chromosomes, and you randomly get one chromosome from each parent.

Fruit flies – Common in Genetics Experiments

• Fruit flies are Drosophila melanogaster• They have 4 pairs of chromosomes• They use the same XX, XY sex determination

as people• They breed quickly (every 2 weeks, hundreds

of offspring)• Wild type is the phenotype most common in

natural populations

Linked Genes

• Genes on the same chromosome tend to be inherited together “linked genes”

So why do offspring look different from parents?

Genetic Recombination

• Genetic recombination is the production of offspring with combinations of traits different from those found in either parent.

• 1. Independent Assortment• 2. Crossing Over

Independent Assortment

• The phenotypes of the parents are called parental types.

• The offspring, with new and different phenotypes, are called recombinant types or recombinants.

• This happens because offspring receive one chromosome from each parent, and end up looking different.

Crossing Over

• In prophase I of meiosis, genes of homologous chromosomes switch around, called crossing over.

• This creates recombinant chromosomes, which makes offspring look different than parents.

Linkage Mapping

• A genetic map is an ordered list of the locations of genes on a chromosome.

Linkage Mapping• Based on a linkage map, one can assume: the

farther apart 2 genes are, the more likely a crossover will occur between them, therefore the recombination frequency is higher.

Linkage Mapping

• A linkage map is a genetic map based on recombination frequencies.

• Units are called map units and show the distance between genes.• 1 map unit = a 1% chance of recombination.• If two genes are 50 map units apart, how likely is recombination?

Cytogenic Map

• A cytogenic map locates genes on chromosomes with respect to visible features like stained bands.

Sex linked traits

• Men are XY, Women are XX• Eggs only carry X, sperm carry X or Y• Significant genes are only carried on the X

chromosomes, which is larger and longer.

Sex-linked genes

• A gene located on the X chromosome is called a sex-linked gene.

• Females ONLY express the trait if they inherit the trait on both chromosomes: XaXa

• If they are XAXA (normal) or XAXa (carrier) they do not have it

• Men ONLY need ONE copy of the recessive gene to have the trait: XaY

• If they are XAY, they don’t have it.

Sex-linked genes

• Therefore, men are much more likely to have a sex-linked trait or disease, they only need one gene to get it, women need two!

• Color-blindness• Baldness• Muscular dystrophy• Hemophilia

Practice: Hemophilia is sex linked.

• Cross a female carrier with a normal male.• XAXa x XAY• If they have a girl, what are the chances she is

a carrier? Normal? Has hemophilia?• If they have a boy, what are the chances he

has hemophilia? Is normal? Is a carrier?

Practice: Hemophilia is sex linked.

• Cross a man with hemophilia with a carrier woman.

• XaY x XAXa

• If they have a girl, what are the chances she is a carrier? Normal? Has hemophilia?

• If they have a boy, what are the chances he has hemophilia? Is normal? Is a carrier?

X inactivation in females

• Sometimes, the second X is inactivated in females, and is called a Barr body.

• She is still normal and fertile, but sometimes has patchy skin.

• This is what causes calico cats, every calico cat you see MUST be female.

Abnormal Chromosome Number• Nondisjunction is when chromosomes do not separate

correctly during meiosis.• This causes an abnormal chromosome number, called

aneuploidy• Trisomy is when you have 3 chromosomes instead of 2 (2n

+ 1)• Monosomy is when you have 1 chromosome instead of 2

(2n – 1)• Polyploidy is having more than one complete set of

chromosomes• If any of the above organisms survive to birth, it will have

major developmental abnormalities

Down Syndrome

• Trisomy 21 is Down Syndrome, where the child has 3 copies of chromosome 21 (instead of 2)

• Different facial features• Short stature• Mental disabilities• Prone to leukemia and Alzheimer’s• Sexually underdeveloped or sterile• Reduced Life expectancy

Aneuploidy of Sex chromosomes

• XXY is Klinefelter Syndrome: male sex organs, small testes, sterile

• XYY: taller than average• XXX: trisomy X, healthy and normal• XO: Turner Syndrome, Monosomy X, sterile,

appear female, no secondary sex characteristics develop without hormone therapy

• All of the above have normal intelligence

Alterations of chromosome structure

• Deletion: chromosomal fragment is deleted

• Duplication: a chromosomal fragment is doubled

• Inversion: chromosomal fragment gets reversed

• Translocation: chromosomal fragments get switched around

Other Diseases

• Cri du chat: deletion from chromosome 5, mentally disabled, die early

• Chronic myelogenous leukemia: 22 and 9 trade pieces, cancer of white blood cells results

2 Exceptions to this chapter

• 1. Genomic imprinting: a zygote expresses only one allele (either mom’s or dad’s) instead of both.

• 2. Inheritance of organelle genes: we have DNA in our mitochondria, and it comes only from mom