Other Laws of Inheritance

• Mendelian Genetics: characteristics controlled by dominant and recessive paired alleles

– Many traits follow the patterns outlined by Mendel’s Laws, many do not.

Incomplete Dominance

• Offspring heterozygous for a trait have an intermediate appearance.

• Neither allele is completely dominant over the other

• Ex: Snapdragon flowers– Homozygous Red x Homozygous White– Results in all PINK flowers

Showing a Cross for Incomplete Dominance– Use a capital letter for each allele since each influence trait

equally

• RR (homozygous red) x WW (homozygous white)• What is phenotype ratio of offspring?

• RW x RW (heterozygous pink)• What is phenotype ratio of offspring?

– How is this ratio different from the offspring

of Mendel’s Monohybrid cross? (Tt xTt)

Codominance• If individual is heterozygous, both alleles

for gene are expressed.

• Ex: Chickens– Black feathers (BB) x White Feathers (WW)– Offspring (BW) are checkered and have both

black and white feathers

• Try a codominant cross:

• Checkered (BW) x checkered (BW)

• Checkered (BW) x white (WW)

• Checkered (BW) x black (BB)

• Ex: Roan Cattle– Offspring of pure red and pure white cows– Have both red and white hair

Multiple Alleles

• A gene that has more than two alleles.

• Each individual can only have two alleles

• Ex: Human Blood Types

Possible Blood Types:

A, B, AB, O

– There are 3 alleles for blood type (A, B, O)

– A and B are codominant alleles (IA, IB)

– O is a recessive allele (i)

Six genotypes are possible for blood:

• IA IA

• IA IB

• IB IB

• IA i • IB i • i i

• What blood types would they have?

• Ex: Type A (homozygous) x Type B

• What are the blood types of the offspring?

• Try a Cross: • What blood types do the offspring have?

• IB IB x IA IA

• IA i x i i

• IA IB x i i

• If a Type A mother and a Type B Father produce a type O child, what are the parent’s genotypes?

– Ex: temperature, nutrition, light etc.

• Ex: Coat color in Himalayan rabbits– Black fur present only on areas of body that are

colder. (ear, nose, feet and tail)

Environmental Influences on Gene Expression

• Ice Pack placed on fur = fur turns black

• If rabbit kept in warm environment, all fur is white.

Genetic Research and Testing

• Genetic Research:– To study genes for different traits it is best to

use an organism that grows and reproduces quickly an produces many offspring.

– Can do controlled breeding experiments– Can do forced “inbreeding”

• Ex: Drosophila melanogaster (fruit fly)

Genetic Tests

• Test Cross: – To determine if an individual is homo or

heterozygous for a trait.– Breed to a homozygous recessive.

– Ex: • Drosophila: red eyes dominant over white.• To see if red eyed fly is hetero/homo, breed to

white eyed fly

Sex Determination• Humans have 23 pairs of chromosomes• 22 pairs are autosomes• 1 pair are sex chromosomes

I’m a BOY! I’m a GIRL!These pictures are called karyotypes!

Sex Determination

• XX = female, XY = male• Males can give Y chromosome or X chromosome• Females can only contribute X• There is always a 50% chance of being boy or girl

Sex Linked Traits• If allele is found on an X or a Y chromosome it is

inherited differently in males and females.

• Ex: – Calico Cats– Color Blindness– Hemophilia– Duchenne Muscular Dystrophy

Calico Cats: (Codominant X Linked Trait)– X chromosome has gene for black or orange

fur (codominant)– Only females can be “Calico” XBlackXOrange

– Males can only be Black or Orange

they have only one X chromosome and one Y

I’m always a girl!

Colorblindness: (Recessive X linked trait)

• Certain colors cannot be distinguished from others (usually red or green)

• Most often seen males

• Females are more likely to be “carriers”

• Father cannot pass gene to son, only mother

Hemophilia: (Recessive X-linked trait)– Blood has trouble clotting

• XHXh Carrier female

• XHY Normal Male

• XhY Affected Male

• XhXh Affected Female

• XHXHNormal Female

• Examples of Crosses for Hemophilia

• Queen Victoria of England passed this gene onto some of the royal family offspring

She must have been a “carrier”

Queen Victoria

Chromosomal Disorders

• Due to the presence of absence of an entire chromosome or part of a chromosome.

• Can be determined by doing a Karyotype from blood or amniotic fluid

• Amniocentesis: – Prenatal test

– Grow cells from amniotic fluid around fetus.

– Helps to rule out major chromosomal abnormalities

Nondisjuction

• Nondisjunction: – homologous

chromosomes fail to separate properly during meiosis.

– Results in egg or sperm with one or more extra chromosomes.

Normal Meiosis Nondisjunction

What is Wrong?

Nondisjunction of Sex Chromosomes

• Klinefelter’s Syndrome (XXY)– 2 X’s from mother, Y from father– Often tall, long arms and legs– May have some gynomastia and small

testicles

• Turner’s Syndrome (XO)– Offspring has only one X (always female)– Usually sterile, short, thick neck

Nondisjunction of Autosomes

• Trisomy 21 (Down’s Syndrome) (47, +21)– Most commonly seen chromosomal disorder

resulting in live birth– Extra Chromosome 21

• Other trisomys can happen as well

• Babies may make it to term but usually don’t live very long (nonviable)– Ex: Trisomy 18, Trisomy 13

Gene Linkage• When alleles for different traits are located

on the same chromosome

• They tend get inherited together.– They are “linked”– They do no “assort independently”

• Ex: Red hair and freckles

Crossing Over and Linked Genes

Crossing Over: homologous chromosomes

line up during synapsis and exchange

material.– Increases variation in offspring.

Sometimes linked

genes will separate

when crossing over

occurs.

The closer genes are

on the chromosome,

the less chance they

will be separated during

crossing over

Autosomal Genetic Disorders

• Caused by defective alleles on autosomes

• Can be recessive or dominant

Sickle Cell Anemia: (Autosomal recessive)

• Red blood cells have a sickle shape

• Very fragile and break easily

• Clump up on blood vessels (causing pain)

• Reduces oxygen carrying capacity of red blood cells

• Caused by a single flaw in gene

• One nitrogenous base in DNA sequence is different

• Cause wrong amino acid in protein chain for hemoglobin, changing it’s shape

• Hemoglobin doesn’t transport oxygen as well

• Mostly found in people of African American descent – (1 in 500 births heterozygous)

• If disorder has negative effects why so common in population?– Malaria is a deadly disease

common in Africa

– Heterozygous individuals for sickle cell have malarial resistance, more likely to survive than people who aren’t carriers.

I spread malaria!

Phenylketonuria (PKU):(Autosomal recessive)

• Enzyme that breaks down amino acid phenylalanine doesn’t function

• Chemical builds up in system and eventually forms substances that can damage the brain and cause mental retardation

• Can test for it at birth and treat with a low phenylalanine diet

Tay-Sachs Disease: (Autosomal Recessive)

• Incurable inherited disorder that damages the brain

• Enzyme doesn’t function that helps breakdown lipid in brain.

• Brain tissue deteriorates

• Most often found in eastern European Jews

• Death occurs several years after birth

• http://www.youtube.com/watch?v=SeoPF74QSms&safe=active

Gene-Chromosome Theory

• Explains the hereditary patterns that Mendel and others observed.

• The genetic characteristics of an organism are determined by the genes for different traits present on their chromosomes.

Mutations

• Sometimes a genetic disorder is caused by a mutation of a gene.

• The mutation can only be passed to the next generation if it happens in a sex cell.

• Mutagenic Agents:– Exposure to x-rays– Radiation – Certain chemical toxins