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) 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?

IB IB x IA IA IA i x i i IA IB x i i 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?

Environmental Influences on Gene Expression 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)

Ice Pack placed on fur = fur turns black If rabbit kept in warm environment, all fur is white.

Genetic Research and Testing 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 These pictures are called karyotypes! I’m a BOY! I’m a GIRL!

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 XHXH Normal 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.

Nondisjunction Normal Meiosis

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