lecture note Nov 6

Nov 6 Chapter 9 Patterns of Inheritance (continued)

Genetics of autosomal dominant traits

- They don’t skip generation.

- If one parent is homozygous for a dominant trait, the probability of their child getting the same trait is 100%.

Parents’ genotypes: AA x AA

AA x Aa

AA x aa

- If both parents are heterozygous for a dominant trait, the probability of their child getting the same trait is 75 %.

Parents’ genotypes: Aa x Aa

genotypic ratio AA : Aa : aa = 1 : 2 : 1

phenotypic ratio dominant : recessive = 3 : 1

- If one parent is heterozygous for a dominant trait and the other parent don’t have the trait, the probability of their child getting the same trait is _______.

Parents’ genotypes Aa x aa

genotypic ratio

phenotypic ratio dominant : recessive =

Genetics of autosomal recessive traits

- They can skip generation.

- Carriers – Individuals who do not show the recessive traits, but carry recessive allele (gene). They are heterozygous.

Example - The child of a health couple has cystic fibrosis because the parents are carriers of cystic fibrosis allele.

- If one parent is heterozygous for a recessive trait and the other parent is homozygous for a recessive trait, the probability of their child getting the recessive trait is __________.

Parents’ genotypes: Aa x aa

genotypic ratio

phenotypic ratio dominant : recessive =

- If both parents are homozygous for a recessive trait, the probability of their child getting the same trait is _______.

Parents’ genotypes: aa x aa

- If both parents are carriers of a recessive trait, the probability of their child getting the recessive trait is _______.

Parents’ genotypes: Aa x Aa

genotypic ratio

phenotypic ratio dominant : recessive =

Human genetic disorders

- Most human genetic disorders are recessive.

- Marriage between close relatives increases the probability of producing children with harmful recessive traits.

Example 1. High frequency of deafness in Martha’s Vineyard is the result of frequent marriage between close relatives.

Example 2. High frequency of hemophiliacs (sex-linked recessive disorder) among royal families in Europe.

- Pedigree (family tree) helps to keep track of family history for the trait.

Example: Inheritance of the deafness gene

- The frequencies of genetic disorders differ in different regions and different population.

Example 1 Frequency of cystic fibrosis

1/17,000 African American

1/90,000 Asian American

1/1,800 Caucasian American

Example 2 Andermann's Syndrome (Read the CBC news article shown in the last page of this lecture note.)

Example problems

1. Dwarfism is a dominant trait. If one parent is dwarf and the other parent is normal height, the probability of their child being dwarf is ________ .

Dwarf allele: DW

normal allele: dw

Note: The dwarf parent is heterozygous for the dwarfism trait. Homozygous genotype is fatal.

Parents’ genotypes: x

Genotypic ratio:

Phenotypic ratio: dwarf : normal =

2. Deafness is a recessive trait. One parent is deaf and the other parent is normal. If their first child is deaf, what is the probability of the next child being deaf.

Deaf allele: d

normal allele: D

Parents’ genotypes: x

Genotypic ratio:

Phenotypic ratio: deaf : normal =

3. Cystic fibrosis is a recessive trait. Neither parents have cystic fibrosis but their child has cystic fibrosis. What are the genotypes of the parents? Use lowercase letter c for the cystic fibrosis allele and uppercase letter C for normal allele. What is the probability of the next child having cystic fibrosis?

Parents genotypes: x

Genotypic ratio:

Phenotypic ratio: cystic fibrosis : normal =

4. Huntington’s disease is a dominant trait. One grandmother had Huntington’s disease but neither parents had Huntington’s disease. What is the probability of the child having the disease?

3) Mendel’s principle of independent assortment (Mendel’s second law of inheritance)

dihybridcross – Cross of parents differ in two traits.

“Each pair of alleles segregates independently during gamete formation.”

True for traits that resides in different chromosomes.

Two traits in pea Seed shape: round (R), winkled (r)

Seed colour: yellow (Y), green (y)

Parents’ traits (genotype): round and yellow (RRYY) x winkled and green (rryy)

Genotypes of gametes: RY ry

F1 generation (genotype): All round and yellow ( RrYy)

Genotypes of gametes: RY, Ry, rY or ry

F2 generation

Genotype ratio RRYY : RrYY : RRYy : RrYy : RRyy : Rryy : rrYY : rrYy : rryy=

Phenotype ratio

Round and Yellow: Round and Green : Winkled and Yellow : Winkled and Green =

Chromosome basis of inheritance

4) Variation in Mendel’s principle

a. incomplete dominance

-Heterozygotes exhibit intermediate phenotype.

ex. colour of snapdragon flower

red flower: RR

white flower: rr

pink flower: Rr

Monohybrid cross of red and white flower parents

P generation

F1 generation

F2 generation

genotypic ratio

phenotypic ratio: red flower: pink flower: white flower =

CBC News: Quebec researchers find gene for brain disease

http://cbc.ca/storyview/CBC/2002/10/08/neuro_gene021008

Quebec researchers find gene for brain disease Last Updated Tue Oct 8 19:12:57 2002

MONTREAL-- Neurologists at McGill University have identified a defective gene that has left hundreds of Quebec children in wheelchairs with a degenerative disorder. The children suffer from Andermann's Syndrome, which results from a mutation that inactivates a gene critical to the development of the nervous system. The gene is associated with the development of a part of the brain that bridges the left and right hemispheres and allows the two sides to communicate. Without it, the developing child will be unable to walk, will spend adolescence in a wheel chair, his mind and body will deteriorate and he will likely die by age 30.

"This is a gene that's not working," said Dr.Guy Rouleau of McGill University's Health Centre Research Institute. "To replace the function of a gene is not an easy thing to do." Most of those afflicted with the disease live in the Saguenay-Lac-St-Jean region north of Quebec City, where one in 2,100 children is born with it, Rouleau said Tuesday. Others cases have been identified in those with Quebec roots in Western Canada, and the disease has also been found in Italy, Turkey, South Africa and Brazil. One in 22 Saguenay residents thought to be a carrier. Researchers believe some of the settlers who colonized the region had the defective gene.

Rouleau, who is credited with the discovery, said parents will now be able to undertake a simple test to determine if they are carriers of the defective gene. If two people who are carriers conceive a child, there's a 25-per-cent chance the child will have the disorder. Since the test of amniotic fluid can be done early in pregnancy, parents may decide to end the pregnancy. Alain Coude's twin daughters, Valerie and Alexandra, suffer from peripheral neuropathy because they inherited the defective gene. He said he hopes the discovery might someday save other families from going through what his family has.

Doctors say the discovery is also significant because it yields important information about the development of the brain and nervous system. The researchers say the publication of their results in the November issue of the journal Nature Genetics may rekindle interest in the area of genetic neuroscience worldwide. Researchers analysed DNA from 191 French Canadian families, including 81 with the disease, before finding the defective gene.

Written by CBC News Online staff