Ch 6.2 Inheritance in Sex Linked Genes.notebook April 19, 2018

Transcription

1 Name 3 "Big Ideas" from our last notebook lecture: * * * 1

2 Inheritance of Linked Genes 2

3 Learning Goals Understand Sex linked Genes How to use a Punnett Square to solve genetics problems and how to use a Pedigree Flowchart to show patterns of inheritance with sex linked genes 3

4 A) Linked Genes Remember... a chromosome is divided into many sections called "genes" and each chromosome may contain up to a few thousand Genes that do not assort independently, but rather assort together, are called "linked genes" Linked Genes are on the same chromosome and tend to be inherited together ("stick together") All of the genes on any one chromosome are called a linkage group because they are inherited together But... linked genes do not always stay linked Scientists have discovered that they segregate on a regular basis due to the process of crossing over that happens in Meiosis Prophase 1 when non sister chromatids exchange pieces of chromosomes 4

5 Crossing over can sometimes "unlink them" Scientists have discovered that the frequency depends on how close the alleles of the linked genes are positioned on a chromosome Crossing over occurs more frequently between alleles that are far apart on a chromosome than between alleles that are closer together End Result: Scientists have discovered that there are fewer gametes with the recombined alleles because crossing over is a random event and it occurs infrequently 5

6 text p252 In most of the gametes formed, there is no crossing over they maintain the linkage of the alleles. In a small minority of gametes, crossing over occurs and alleles of previously linked genes become unlinked. 6

7 B) Sex Linked Inheritance Disorders Traits that are controlled by genes on either the X or Y chromosome, sex chromosome pair #23, are called sex linked traits because they are linked to the genes that determine sex They are identified by their different rates of appearance between males and females Sex linked inheritance is therefore the passing down of genes that are on the X or Y chromosomes A gene located on the X chromosome is called X linked and a gene located on the Y chromosome is called Y linked 7

8 Interesting Difference: The X and Y chromosomes, although paired together during meiosis and for karyotyping purposes, have very little homologous DNA The X chromosome has about 2000 genes, while the Y chromosome has fewer than 100 The most important genes on both are the sex determination genes... but all the other genes for other traits have two copies on the X chromosome to only 1 copy on the Y chromosome Because there are more genes on the X chromosome, there are more mutation opportunities therefore... there are more X linked inheritance traits than Y linked inheritance traits AND... because the male only needs to inherit one X allele to be affected while the female needs to inherit both X alleles to be affected... there are more males affected by X linked recessive traits! 8

9 Punnett Square X Linked Recessive Disorder father X A Y X a mother X a X A X a X A X a X a Y X a Y So this represents a father who is Normal and a mother who has the Recessive Disorder every pregnancy only has the males (sons) inheriting the disorder. 9

10 text p255 Most Common in Humans: 10

11 Recall, that there are also other sex linked disorders that involve missing or the addition of an X or Y chromosome... Text p179 11

12 text p256 Highlight #1: Hemophilia (Recessive X Linked Disorder) Video 1: Amoeba Sisters explain Hemophilia Video 2: Canadian Hemophiliac Society Highlight #2: Red/Green Colour Blindness (Recessive X Linked Disorder) Video 3: Colour Blindness Explained 12

13 text p254 Eye colour in fruit flies is also sex linked.... we will be doing an on line Fruit Fly Lab next week :) 13

14 What is a Barr Body? A Barr Body affects every female; it is not a disorder, but just a phenomenom that affects the "X" somatic cell chromosome: Question?: Since females carry two X chromosomes and males only carry one, why is there no difference in the expression of X linked genes between males and females? Answer: The answer is that in every mammal cell (except marsupials) there is only one functioning X chromosome Females have 2 copies of the X chromosome in every cell, but scientists have discovered that one X chromosome in each female cell has become inactive... it is called a Barr Body at an early stage of embryonic development, an X chromosome in each female cell is deactivated which X chromosome is deactivated is completely random in every cell Example: Calico Coat Colour in cats: In heterozygous females, roughly 50% of the cells have an active X chromosome with the allele for black coat colour, and 50% of the cells have an active X chromosome with the allele for orange coat colour this results in a tortoiseshell coat with patches of both black and orange the patches of white are the result of the interaction with a different gene. It is like "co dominance" but only on the X chromosome! Barr Body Videos: body definition formation.html 4zdUF4uE 14

15 Ch 6.2 Inheritance in Sex Linked Genes.notebook 6.3 The Human Genome Project Text p The 13 year ( ) Human Genome Project:... determining the DNA sequence of the human genome is considered to be one of the most pivotal contributions to Science ever made! It was the foundation for today's continually changing genetics research and discoveries and advancing biotechnology! Video 4: Hank talks about Genome Project 15

16 Let's read together textbook page

17 Let's try... Practice Problems p258 17

18 Review & Learning Check Read p Answer Q's 2,7 10,13,15 p259 18