Genetics – yeah!

Chapter 14

Important Terms

• Gene vs allele

• Genotype vs phenotype

• Homozygous vs heterozygous

• Probability

Mendel and his pea plants

Mendel had many different lines of pure breeding pea plants.

•Pure breeding – always giving the same offspring.

Purple X White

All Purple

P1 generation

F1 generation

Purple F1 X Purple F1

Purple and White

3 : 1 Ratio

F2 generation

Other Traits Mendel Looked At

The Four Components of Mendel’s Hypothesis

• There are alternative forms for genes, the units that determine inheritable characteristics:

– The gene for pod color existed in two alternative forms, one for green and one for yellow.

– Alternative forms of a gene are now called alleles.

• For each inherited characteristic, an organism has two alleles, one inherited from each parent.

– Mendel's experiment included on parental variety which had a pair of alleles for green pod color and one which had a pair of alleles for yellow pod color.

– The F1 hybrids inherited from the parental plants one allele for green pod color and one allele for yellow pod color.

• A sperm or egg carries only one allele for each inherited characteristic, because allele pairs separate (segregate) from each other during the production of gametes. At fertilization, the sperm and egg unite with both contributing their alleles. This restores the gene to the paired condition. – In Mendel's experiment, each gamete of a parental plant carried

one allele for pod color, specifying either green or yellow. – Cross-pollination to produce the F1 resulted in the combination

found in this generation.• When the two alleles of a pair are different, one is fully

expressed and the other is completely masked. These are called the dominant allele and recessive allele, respectively. – Mendel's F1 hybrid had green pods, which was recessive.

What is a test cross?

A cross with a known homozygous recessive.

On March 7, 1964, Pierre born in a remote part of Quebec Province in Canada.

•He appeared to be a healthy six-pound twelve-ounce child, except he did not eat well.

•He became progressively more lethargic, vomiting periodically. Most peculiarly, his urine smelled of rotten cabbage, and soon the smell permeated his clothes and body.

•Admitted to the hospital on September 14, his muscles were weak and his ribs were showing.

•He gained weight and strength for a while, then suddenly took a turn for the worse.

• On November 30, baby Pierre died.

Became increasingly apparent that other babies in the Chicoutimi area of Quebec Province had similar symptoms

people recalled similar deaths in this remote area 120 miles north of Quebec City.

Some families lost several children to Pierre's disease.

In those families, it soon became clear that the parents were normal, but about one quarter of their children were afflicted.

Boys and girls were equally afflicted.

Specialists soon concluded that all of the facts indicated that this was a genetic disorder.

• Baby Pierre and the other stricken children were victims of hereditary tyrosinemia.

• This is caused by an autosomal recessive disease.– The children lacked the normal gene which produces

a liver enzyme that breaks down the amino acid tyrosine. Without the enzyme, tyrosine builds up in the liver and kidneys leading to the cabbage-like smell of the urine. Lethal side-effects follow.

• Both parents avoid this fate because, although they carry one copy of the defective gene, they carry a normal gene which produces more than enough enzyme for normal liver function. The parents are unwitting carriers of the disease. In genetic terminology they are heterozygotes, while Baby Pierre was a homozygous recessive.

Below is a pedigree of three generations of Canadians.

Which people have the disease? What is their genotype?

Which people must be carriers of the disease? What is their genotype?

Which people can we not determine a genotype for?

A = Dominant

a = recessive

1. What is the likelihood that female K is a carrier? 2. What is the likelihood that female K will have a normal child

if she marries a normal person who is a carrier for tyrosinemia?

Hereditary tyrosinemia is usually quite rare, affecting only 1 in 100,000 newborns. The situation in the French Canadians in Chicoutimi, Quebec, is dramatically different; 1 birth in 685 can be expected to produce a child with the disorder.

• From the answers below, what is the most likely reason for the large number of tyrosinemia cases in Chicoutimi. Explain the reason for your answer.

(A) A high mutation rate.(B) A selective advantage for tyrosinemia in this part of

Quebec.(C)Founder effect – initial population had an unusually high

number of carriers.(D)Nutritional patterns in the people; large quantities of

tyrosine in the diet.(E) Pleiotropy where the allele for tyrosinemia has

beneficial effects as well as harmful effects.

Crosses of multiple traits

Must look at all combinations of gamete production

Probability and Genetics

• Chance that an offspring will receive a certain allele.

Case #2 Love and Luck

• Greg and Olga have relatives who are hemophiliacs. Greg has some relatives that suffer from myotonic dystrophy. Olga had a friend die from cystic fibrosis.

• They are married and considering having a family, but want to know what the chances are that their offspring might have hemophilia, myotonic dystrophy, or cystic fibrosis?

Draw a pedigree showing what we know about Greg.

Draw a pedigree about what we know about Olga.

Myotonic Dystrophy• Autosomal Dominant Trait

– Carried on the non-sex chromosomes– b/c dominant they are always shown

• Questions• Why do autosomal dominant disorders not skip

generations?• Could Greg or his mother be carriers of the gene that

causes myotonic dystrophy?• Is there a possibility that Greg’s aunt or uncle is

homozygous for the myotonic dystrophy (md) gene?• Symptoms of myotonic dystrophy sometimes don’t show

up until after age fi fty. What is the possibility that Greg’s cousin has inherited the md gene?

• What is the possibility that Greg and Olga’s children could inherit the md gene?

Cystic Fibrosis• Autosomal recessive trait

– Must have two copies of recessive allele to get disease.

– People can be carriers of one recessive allele and not show the disease.

Questions:

What does consanguineous mean?

Why is this concept especially important when discussing recessive genetic disorders?

HemophiliaSex-linked recessive trait (X chromosome )• What are the characteristics of X-linked recessive

inheritance?• Why does a son never inherit his father’s defective X

chromosome?• What is required for a woman to display a sex-linked

recessive trait?• Return to the pedigree drawn earlier for Greg and Olga;

mark those persons who are carriers of the factor viii deficiency gene.

• What is the chance that Olga carries the gene for factor viii defi ciency? Calculate the probability that she will pass it to her off spring. Will male children be affected in a different way than female children?

• What is the chance that Greg carries the factor?

Colorblindness

• Bob is color-blind and so is his brother Rob.

• Bob’s sister (Bobette) is not colorblind, neither is his mom (Bobbi) or dad (Robert).

• None of Bob’s grandparents are colorblind and neither are his 2 daughters. His brother’s daughter is not colorblind. One of his sister’s children is colorblind.

• What do we know?

Co-dominance

• Blood Types– Three alleles– IA, IB, Io

• How are blood types determined?