Campbell: Chapter 15

CONCEPT 3: APPLYING THE CHROMOSOMAL BASIS OF INHERITANCE TO ANALYZE THE EFFECTS OF SEX CHROMOSOMES, LINKED GENES, AND RECOMBINED RECOMBINES GENES

Campbell: Chapter 15

Learning Intentions

Goal: to analyze mechanisms of chromosomal inheritance

You must know: How the chromosome theory of inheritance

connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance

The unique pattern of inheritance in sex-linked and linked genes

Remember...Chromosome Theory of Inheritance

What is the relationship between a pair of alleles and a homologous pair of chromosomes?

Remember...Chromosome Theory of Inheritance

Alleles are located on chromosomes. The homologous pair of chromosomes

code for the same genes, each having one allele.

Homologous chromosomes

Remember...Chromosome Theory of Inheritance

Since genes have specific locations on chromosomes (loci), when chromosomes segregate and assort independently so do the genes!

Sex-Linked Genes

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

Would this individual be a male or a female? How do you know?

Morgan’s Sex-Linked Discovery in 1910!

He got a Nobel Prize... He’s kind of a big deal.

Thomas Hunt Morgan 1866-1945

The Fly Room!

The gene for fly eye colour is found on the “X” sex chromosome!

Sex-Linked Genes – Checking In... Can Fathers pass sex-linked genes to

their sons?

For males, do the terms homozygous and heterozygous apply?

Do most genes on the “X” sex chromosomes haveto do with sex characteristics?

Sex-Linked Genes

NO. Fathers pass sex-linked genes to their daughters but not their sons.

NO. For males, the terms homozygous and heterozygous do not apply for sex-linked genes since males only have one copy of the gene.

NO. Most genes on the “X” sex chromosomes havenothing to do with sex characteristics.

Colour Blindness: Sex-Linked Gene

Can you read the number?

Colour Blindness: Sex-Linked Gene

What happens if you cross a normal female with a colour blind male?

Colour Blindness: Sex-Linked Gene

All females are carriersAll males are normal

Colour Blindness: Sex-Linked Gene

What happens if you cross a carrier female with a normal male?

Colour Blindness: Sex-Linked Gene

½ females are carriers½ females are normal ½ males are colour-blind½ males are normal

Colour Blindness: Sex-Linked Gene

What happens if you cross a carrier female with a colour-blind male?

Colour Blindness: Sex-Linked Gene

½ females are colour blind½ females are carriers ½ males are colour blind½ males are normal

Colour Blindness: Sex-Linked Gene

All females are carriers

½ females are carriers and ½ males are colour-blind

½ females are colour blind, ½ females are carriers. ½ males are colourblind

Other examples of Sex-Linked Genes:

Duchene Muscular Dystrophy Progressive muscle weakening,

loss of coordination

Hemophilia Reduced ability to clot blood

Try This!

Neither Tom nor Rhonda has muscular distrophy, but their firstborn son has it. What is the probability that a second child

will have this disease?

What if the child was a boy? A girl?

Try This!

Neither Tom nor Rhonda has muscular distrophy, but their firstborn son has it. What is the probability that a second child

will have this disease?

What if the child was a boy? A girl?

Try This!

Neither Tom nor Rhonda has muscular distrophy, but their firstborn son has it. What is the probability that a second child

will have this disease? 1/4

What if the child was a boy? 1/2 A girl? 0, but ½ carrier

X-Inactivation

Since females get two copies of the “X” sex chromosomes, they need to inactivate one of them! Done by adding methyl groups to DNA...

Bunches up the chromosome

Happens randomly in each cell when the female is an embryo

Inactive chromosome is called a Barr body

Example of X-Inactivation: Calico Cats!

The kitten on the left “CC” is an identical clone of the cat on the right “Rainbow”. How can you explain the different colouring?

http://scientificcuriosity.blogspot.com/2006/09/that-cute-kitten-is-copycat-or-cc-first.html

CC (copy cat) was born in 2001. She is the first cloned cat.

Discuss...

During early embryonic development of female carriers for colour blindness, the normal allele is inactivated by chance in about half the cells. Why, then, aren’t half of female’s colour blind?

Discuss...

During early embryonic development of female carriers for colour blindness, the normal allele is inactivated by chance in about half the cells. Why, then, aren’t half of female’s colour blind?

Eye cells must come from multiple cells in early embryo

Having ~1/2 of mature eye cells expressing proteins for colour detection must be enough for normal colour vision

Try This!

Handout with questions... “Sex Linked Practice”

Chapter 15!

Next Class: Linked Genes