CHAPTER 14: MENDEL AND THE GENE IDEA

• Gregor Mendel - ~1857 grew peas and discovered patterns in inheritance

• Gene - a specific sequence (section) of DNA that codes for a protein

• Allele - the form that a gene takes (for example: dominant [B] or recessive[b])

• Heterozygous (hybrid or carriers) - different alleles for a trait

• Homozygous (pure) - same alleles for a trait• Genotype -“type of genes”; the combination of

genes that an individual has for a trait• Phenotype - the “appearance” of the trait • Law of Segregation - 2 alleles for a heritable

trait separate in gamete formation

• Dominant and Recessive – Ex. Round allele (R)in peas makes round seeds due

to enzyme that converts sugar to starch (starchy peas take in more water and fill out) and wrinkled peas (r) make dysfunctional enzyme• Rr – makes both enzymes, but enough working

enzyme to be round– Ex. Tay sachs is a recessive disease which cannot

make enzyme to break down fat (causes death)• Tt – has intermediate levels of good enzyme

compared to TT and tt• Norm of reaction – range of phenotypic possibilities

• Ex. Blood type is concrete, but red blood cell count is not

• Nature vs. nurture

• Types of problems:

– Single trait (P, F1, and F2 generations)

– Test crosses (cross an unknown with recessive)– Two traits– Incomplete dominance - heterozygote’s

appearance somewhere between phenotypes of two alleles

– Codominance -both alleles separately manifest in phenotype

– Multiple alleles (blood type)– Epistasis– Pedigrees

• Rule of Addition - The probability of an event that can occur in two or more different ways is the sum of the separate probabilities of those ways.

• Rule of multiplication - The probability of 2 or more independent events is the product of the individual probabilities.

• Epistasis - when a gene at one location alters the phenotypic expression of another gene– Example: one gene for hair color and another

gene for depositing the color or pigment)

• Pleiotropy - one gene affects an organism in many ways– Example: the gene that causes cystic fibrosis

causes multiple symptoms• Qualitative characteristics – vary along a

continuum – Example skin color and height

• Polygenic inheritance - an additive effect of 2 or more genes on a single phenotype– Example: skin color

Genetic Testing

• Amniocentesis– Test can be performed starting at the 14th week of

pregnancy.– Needle used to take sample of amniotic fluid

(through abdomen)– Must wait 2 weeks for result– Karyotype of fetus shows chromosomal disorders

• Chorionic Villi Sampling– Test can be performed starting at the 8th week of

pregnancy.– Tube used to take sample (through cervix).– Results in 24 hours– Karyotype of fetus shows chromosomal disorders

Genetic Diseases• Cystic Fibrosis (recessive)

– 1 out of 25 people of European descent are carriers– Most common lethal genetic disorder

• Sickle-cell anemia (recessive) – 1 out of 400 African Americans have disease and 1 out of 10 are

carriers– Aa – have both normal and sickle cells

• Achondroplasia (dominant)– 1 out of 25,000 people are dwarfs

• Huntington’s (dominant)– 1 out of 10,000– Lethal around 30 – 50

• Multifactorial disease – many genes involved in some diseases– Cancer, heart disease, diabetes, mental health disorders

• Mating of close relatives increases chance of 2 rare recessive alleles coming together

Single Trait Crosses

1. Cross two heterozygous brown-eyed individuals. Brown eyes are dominant to blue eyes. What are the genotype and phenotype ratios?

2. Sickle cell anemia is a recessive disease. A woman who is normal marries a man who has sickle cell. The woman has a mother who has sickle cell and so they want to know the chance of having a child with sickle cell anemia?

3. Two normal people (Henry and Amy) both have a sibling who is an albino even though their parents are all normal. A recessive gene causes albinism. What is the probability of these Amy and Henry having a child who is an albino?

Pedigrees

Test Cross

1. If black rabbits are dominant to white rabbits. A breeder has a black rabbit and wants to know the genotype. Show a test cross that would help the breeder determine the black rabbit’s genotype.

Two Trait Crosses

1. Cross a man who is heterozygous for both brown eyes and tongue rolling with a woman who is the same. Brown eyes are dominant to blue and rollers are dominant to nonrollers. What is the phenotypic ratio for this cross?

2. Cross a heterozygous brown nonroller with a homozygous blue nonroller. What is the phenotypic ratio for this cross?

3. Cystic fibrosis is caused by a recessive gene. Albinism is also caused by a recessive gene. Cross a woman who is heterozygous for cystic fibrosis and an albino with a man who is homozygous for no cystic fibrosis and heterozygous for albino. What are the chances of the following?

AaFf? An albino child with cystic fibrosis? aaFF?

Incomplete Dominance

1. Hair shows incomplete dominance. The heterozygous condition is wavy hair. The 2 homozygous conditions are straight and curly. Cross a straight hair woman with a waxy hair man. What is the phenotypic ratio for this cross?

Blood Type

1. Cross a person who is homozygous for type A with a person who is heterozygous for type A. What is the phenotypic ratio for this cross?

2. Cross a person who is heterozygous for type A with a person who is heterozygous for type B. What is the phenotypic ratio for this cross?

Epistasis

1. In rats, black is dominant to brown. Another gene controls the deposition of pigment. Pigment deposition is dominant to no pigment deposition (albino). Cross rats who are heterozygous for both traits. What is the phenotypic ratio for this cross?

Probability Shortcuts

1. What is probability that the following parents will produce an offspring who is AABbCC? 

AABBCc x AaBbCc

BLOOD TYPES

GENOTYPESANTIGENS (MARKERS)

ANTI-BODIES (FIGHTERS)

RECEIVE FROM?

DONATE TO?

TYPE A          

TYPE B          

TYPE AB          

TYPE O