GENETICS

(the scientific study of heredity)

Background information

A chromosome is a linear sequence of DNA segments. Each segment of DNA at a certain locus codes for a particular trait (characteristic). This locus is called an allele. (There are approximately 175,000 alleles on one human chromosome.)

Ways Characteristics are Inherited

1. Monohybrid cross: involves one pair of alleles at some location on one pair of homologous chromosomes.

1. dominant allele: shows itself if present. (D)
2. recessive allele: will not show itself if D is present. (d)

EXAMPLE: tongue rolling (ability to roll is dominant over inability to roll)

Possible phenotypes:

• Homozygous dominant (alleles alike): TT (tongue roller)
• Heterozygous dominant (alleles different):Tt (tongue roller)
• Homozygous recessive (alleles alike): tt (inability to roll)

Before you can determine the progeny (offspring), meiosis must occur in the parents, followed by fertilization.

1. Problem solving: In genetics, we use a method of determining possible progeny called the Punnett square. In order to set up the Punnett square, we must determine all the different possible alleles that can be contributed to the progeny by each parent.
2. Setting up the Punnett square:

1. all the different possible alleles from meiosis occurring in the female parent across the top
2. all the different possible alleles from meiosis occurring in the male parent down the side
3. the blocks making up the square represent possible, not real progeny

EXAMPLE: Tongue Rolling

Phenotype: characteristic visible in progeny (roll or not roll). physical appearance.

Phenotypic ratio: 3:1

Genotype: description of alleles (hetero. tongue roller, homo. tongue roller, homo. non roller). genetic makeup

Genotypic ratio: 1:2:1

Sample Problem:

In guinea pigs, black color dominates white. Determine the phenotypic and genotypic ratios of all progeny of the F₁ generation, if the father has homozygous black hair and the mother is homozygous white. Also, find the F₂ generation and the back cross generation.

Steps in problem solving:

1. set up symbols
2. use symbols to express mom and dad allelic condition (genotypes)
3. determine all the possible different male or female alleles present in sex cells following meiosis
4. set up the Punnett square, putting female possible alleles across the top and male possible alleles down the side.
5. determine possible progeny
6. determine phenotype and ratio (appearance)
7. determine genotype and ratio (genetic description of alleles)

1. Finding F₁ generation (possible progeny from crossing two pure parents for opposite traits)
2. Finding F₂ generation (possible progeny from crossing 2 progeny of the F₁ generation)
3. Finding test cross generation (possible progeny from crossing g F₁ generation back to homozygous parent)

c. Problem solving: Pedigree. genotypic or phenotypic data of genetic patterns on a diagramed chart.

Sample Problem: Study the chart. How is the pedigree trait (straight hairline) illustrated below inherited? Is it a dominant or recessive trait?

2. Dihybrid cross: involves two pairs of alleles on two separate pairs of homologous chromosomes.

EXAMPLE: Guinea pigs: black hair dominates white; curly hair dominates straight.

Sample problem: Cross a male heterozygous black haired guinea pig with straight hair to female white haired with heterozygous curly hair.

1. Non-dominance: an intermediate condition. both members of the pair of alleles express themselves ( a blending of the two traits results) (note: use different lower case letters for the problem since there is no dominance)

Sample problem: cross a short horn white bull with a short horn red cow.

Sample problem: cross a roan bull with a white cow.

2. Multiple Births: Identical twins arise from one fertilized egg. may be in one or two embryonic sacs. Fraternal twins arise from two fertilized eggs. always in two different embryonic sacs. (incidence of fraternal twins increase with age of mother)
3. Sex determination: (in mammals). determined by a pair of chromosomes called the sex chromosome. In man, 22 pairs of chromosomes are the autosomes which have nothing to do with sex determination. XX: female in humans. chromosomes are homologous. XY: male in humans. chromosomes are not homologous. each baby has a 50/50 chance of being either male or female when it is conceived.
4. Sex Linkage: alleles are carried on the sex chromosome. X-linked: alleles carried on the X chromosome with no homologue on the Y chromosome. (approximately 60 have been identified)

EXAMPLE: color blindness, hemophilia (recessive). Duchenne Muscular Dystrophy

Sample:

1. Color blindness (red-green) is recessive to normal color vision. Cross a color blind male to a homozygous normal female.
2. Cross a normal male with a heterozygous normal female.

Y-linked: alleles carried on Y chromosome (in man, only 1 has been identified: hairy earlobes)

1. Sex- Limited: Alleles can express themselves in one sex only, not in both, because of hormonal or anatomical differences in the sexes. These alleles are not located on the sex chromosomes, but on the autosome.

EXAMPLE: Bulls have many alleles for milk production which can be transmitted to their female offspring where they express themselves. However, in sons, the alleles cannot express themselves.

Sample Problem: Cock feathering (fancy feathers) is sex limited to the males only and is recessive to normal feathers. Genotype: XYff

Sample Problem: Premature baldness is sex limited to males only and is dominant over non-balding.

Genotypes: XYBB, XYBb

(Note: in man, sex-limited alleles are primarily responsible for the secondary sex characteristics, such as beards in males. Females have the same number of hair follicles in an area as males, but they develop more in males.)

2. Sex Influenced: Occurs because of certain hormones within the individual sex. These alleles are located on autosomes, not on sex chromosomes.

Sample Problem: Pattern baldness: sex influenced which expresses as dominant in males and recessive in females.

b: non bald (dominant in F, recessive in M)

b': bald (dominant in M, recessive in F)

(Note: baldness may also be caused by environmental factors, such as exposure to radiation, having a disease, having a thyroid deficiency)

3. Environmentally Influenced: alleles expression is influenced by the environment.

EXAMPLES: Fat (food intake), Skin color (sunlight), Strength (exercise)

Note: an allele must be present for the expression of the trait before the environment can influence it)

4. Multiple alleles: More than one pair of alleles for a trait in the whole population, but each individual receives only one pair.

EXAMPLE: Blood type (3 alleles in the population: A, B, O)

Each individual inherits only two of the three alleles and the two the individual inherit express themselves completely.

Sample problems:

1. What type of children would a person be expected from a man with type O blood and a woman with type AB blood?

2. Cross person with A type blood with a person of O type blood.

MUTATIONS

(changes in either chromosomes or alleles)

1. Chromosomal

1. Non-disjunction in sex chromosomes: the sex chromosomes do not separate in meiosis, so the resulting zygote gets either 3 sex chromosomes or 1 sex chromosome instead of the normal 2.

EXAMPLES: XXY: male. does not mature sexually. Klinefelter's Syndrome. (12/10,000)

XO: female. does not mature sexually. Turner Syndrome. (1/10,000)

2. Non-disjunction in autosomes: the autosomal chromosomes do not separate in meiosis, so the resulting zygote gets 3 autosomal chromosomes instead of the normal pair.

EXAMPLE: Downs Syndrome: 47 chromosomes. 21st pair has 3 chromosomes. abnormal mentally and physically. 7,000 per year in U.S. Mostly found in mothers over 35.

(note: no person ever found with fewer than 44 chromosomes. this appears to be the number necessary for human life)

1. Allelic (point mutations)

1. Substitution

EXAMPLE: Sickle cell anemia. Protein hemoglobin has one amino acid substituted incorrectly, thus hemoglobin does not form normally.

2. Deletion

EXAMPLE: Aniridia. Fissure in eye

3. Insertion

(Note: somatic mutations are not passed on to the next generation, but reproductive cell mutations are. Occasionally, mutations are helpful, such as in seedless grapes, large strawberries, etc.)