Chapter 9Patterns of Inheritance

9.8 Pedigree analysis A pedigree can be used to analyze traits in

humans

9.9 Disorders determined by a single gene AUTOSOMAL RECESSIVE DISEASESAlbinismCystic fibrosis PKU (phenylketonuria)Sickle cell disease Tay- Sachs disease

Practice problemMary and Steve have a child with cystic

fibrosis but neither of them have the disease. What are the chances that their next child will have cystic fibrosis?

Ann is a carrier for sickle cell anemia. Her husband is not a carrier. Neither has any symptoms of the disease. What are the chances that their children will have sickle cell anemia?

Autosomal dominant disorders AchondroplasiaHuntingdon’s disease

Problems Peter’s father has Huntingdon’s disease. His

mother does not. Peter’s much older sister also does not have the disease. What are the chances that Peter has Huntingdon’s disease?

Problems Alicia and Phil are both achondroplasics.

What are the chances that they will have a normal stature child? What are the chances they will have a child with achondroplasia?

Mendel's assumptionsAll traits are determined by two alleles (or

one pair of genes)There are two possible genes or alleles for

that traitOne gene is completely dominant to the

other.One gene pair determines one traitGenes for different traits independently

assort (or one gene per chromosome)

Exceptions to Mendel’s assumptions Multiple alleles- traits may be determined by more

than two alleles- blood typeBlending and incomplete dominance- genes may

not be completely dominant – hair textureCo-dominance – both alleles contribute to phenotype

and do not interfere with each other –blood typeLinkage – genes on same chromosome are inherited

together (sex linkage)Epistasis/polygenic inheritance- one trait

determined by more than one gene pair – skin colorPleiotropy- one gene has multiple effects – sickle cell

9.12 Blending/Incomplete Dominance Heterozygous individuals are a blend of two

alleles. For example crossing a red and a white flower generates all pink flowers.

Cross two pink flowers.

Incomplete dominanceCross two snapdragon plants that are

heterozygous for pink flowers and tall height. Tall and red are dominant traits.

9.13 Blood type alleles Three alleles for blood type: A, B, oAlso written IA, IB, IoA and B are codominant, both are dominant

to oWhat are the possible combinations of the

two alleles for blood type?How many blood types exist?

Blood type problems A woman has blood type B and her husband

has blood type A. They have a baby with blood type O. What other blood types are possible in their future children?

Blood type and paternityA woman with Type O blood has a baby with

Type B blood. Two men claim responsibility for the child. Man #1 has blood type A. Man #2 has blood type AB. Who cannot be the child’s father?

9.14 Pleiotropy One allele can have many phenotypic effects

9.15 One trait controlled by multiple gene pairsPolygenic inheritance

Epistasis

Epistasis problemIn corn the dominant allele (I) creates colorless

kernels while the recessive allele (i) allows color to be deposited. At a different gene locus the domina t gene (P) creates purple kernels, while the recessive gene (p) creates red kernels.

What is the color of each kernel?IiPpIippiiPp

Problems Cross two corn plants heterozygous for both

genes

Sex linkage Sex linked genes are on X chromosome and

have nothing to do with genderFemales inherit two copies of the geneMales only inherit one – they only have one XSex linked recessive diseases are more

common in males

Sex – Linkage Problems Color blindness is a sex linked recessive

disease. A woman whose father was color blind marries a normal man. Find the phenotypic ratios in their children

A hemophiliac man and a normal woman marry and have children. What percentage of their children will also have hemophilia?

Sex- Linkage problems Fur color in cats is sex linked. Two co -

dominant alleles for fur color exist in tabby cats, orange and black. Tortoise shell cats have both alleles.

Cross a black female with an orange maleCross a tortoise shell female with a black

male

Are there ever any male tortoise shell cats?

HemophiliaLack a blood clotting proteinRoyal disease- Queen Victoria was a carrier.

Victoria’s daughters spread the disease to other European royal families through marriage. Alexis who was the son of the last czar of Russia was terribly afflicted. His parents preoccupation with his illness was a factor in the Russian revolution

Hemophilia Little to no treatment before the 1950s-

Crutches and wheelchairs for protection, risk of major tissue damage from bleeds

Doctors isolated the missing clotting protein and are able to collect it from donated blood.

All blood is tested for infectious disease because of a highly publicized hemophiliac named Ryan White who died of AIDS from a transfusion

Duchenne Muscular dystrophyUntreatable progressive muscle wasting

diseaseFatal usually in early 20s due to impairment

of heart and breathing muscles