Human Genetic Analysis

Chapter 11

1.  Complex inheritance of traits does not follow inheritance patterns described by Mendel.

2.  Many traits result from alleles with a range of dominance, rather than a strict dominant or recessive relationship.

3.  Gene expression is often related to whether a gene is located on an autosome or on a sex chromosome.

Codominance

! When two dominant alleles are expressed at the same time, both forms of the trait are displayed.

!  Different from incomplete dominance because both

traits are displayed.

Example: red flower x white flower =

red and white flowered offspring equal number of red and white flowers

Example: Black horse x white horse = roan coat equal number of black and white hairs

Intermediate Traits

"  Incomplete dominance - an individual displays a trait that is intermediate between the two parents.

Example: red flower x white flower = pink flower "  Neither the red nor the white allele is completely dominant

Multiple Alleles !  Genes with three or more alleles are said to have

multiple alleles. !  When traits are controlled by genes with multiple alleles, an

individual can have only two of the possible alleles for that gene.

Example: Blood types in humans; hair color; eye color ! Three different alleles –IA, IB, and i result in four

different blood phenotypes – A, AB, B, and O.

X-Linked Traits

" The trait is carried by females ONLY on the X chromosome through a recessive allele.

" Males will HAVE the disorder because they only have one X chromosome.

" The X chromosome has many genes, some of which cause genetic disorders.

Traits influenced by the Environment

" An individual’s phenotype often depends on conditions in the environment.

Example: fur color in Siamese cats # Dark fur around cooler parts of the body- ears, nose,

paws, and tails. Example: skin tone in humans

# Exposure to the sun alters the color of the skin.

Mutations " Changes in genetic material.

" The harmful effects produced by inherited mutations are called genetic disorders.

" Many mutations are carried by recessive

alleles in heterozygous individuals. “carriers”

Genetic Counseling

o  Genetic counseling is a form of medical guidance that informs people about genetic problems that could affect them or their offspring.

1. Genetic Abnormality" Rare, uncommon version of a trait.

2. Genetic Disorder" An inherited condition that sooner or

later will cause mild to severe medical problems.

3. Syndrome" A recognized set of symptoms that

characterize a given disorder.

4. Disease" Illness caused by infectious, dietary, or

environmental factors, NOT by inheritance of mutant genes.

Disorder   Mutation   Chromosome  

Color blindness   P   X  

Cystic fibrosis   P   7q  

Down syndrome   C   21  

Hemophilia   P   X  

Klinefelter syndrome   C   X  

Sickle-cell disease   P   11p  

Tay–Sachs disease   P   15  

P – Point mutation, or any insertion/deletion entirely inside one gene D – Deletion of a gene or genes C – Whole chromosome extra, missing, or both T –Trinucleotide repeat disorders: gene is extended in length

PEDIGREE CHARTS

© 2007 Paul Billiet ODWS

What is a pedigree chart?

" a chart of the genetic history of a family over several generations.

" used to study the transmission of

hereditary conditions

© 2007 Paul Billiet ODWS

Constructing a Pedigree

" Female

" Male

Connecting Pedigree Symbols

" Married Couple

" Children

Symbols used in pedigree charts

" Affected

" X-linked

" Carrier

" Deceased

© 2007 Paul Billiet ODWS

. .

Symbols used in pedigree charts

"  Normal male "  Affected male "  Normal female "  Affected female

© 2007 Paul Billiet ODWS

Organizing the pedigree chart

"  A pedigree chart of a family showing 20 individuals

© 2007 Paul Billiet ODWS

Organizing the pedigree chart

# Generations are identified by Roman numerals.

I

II

III

IV

© 2007 Paul Billiet ODWS

Organising the pedigree chart "  Individuals in each generation are identified by Arabic numerals

numbered from the left "  Therefore the affected individuals are II3, IV2 and IV3

I

II

III

IV

© 2007 Paul Billiet ODWS

Interpreting a Pedigree Chart

1.  Determine if the pedigree chart shows an autosomal or X-linked disease.

!  If most of the males in the pedigree are affected and

the carriers are female, THEN the disorder is X-linked. !  If it is a 50/50 ratio between men and women, THEN

the disorder is autosomal.

Example 1 of Pedigree Charts

$  Is it Autosomal or X-linked?

Answer

$ Autosomal

2.  Determine whether the disorder is dominant or recessive.

!  If the disorder is dominant, one of the parents must

have the disorder. !  If the disorder is recessive, neither parent has to have

the disorder because they can be heterozygous.

Example 2 of Pedigree Charts $ Dominant or Recessive?

Answer

$ Dominant

Example 3 of Pedigree Charts " Dominant or Recessive?

Answer

" Recessive

% Several genes can influence a trait- polygenic

trait. % Determining the effect of these genes is

difficult.

Example: A horse with red hair mates with a horse with white hair, and their offspring has both red and white hair.

How can this be?