Lab 18: human mendelian genetics

Character

A heritable feature of an organism that varies among individuals, features such eye or hair color

Trait

A variety or state of a character, such as a blue or green eye color or no freckles

Structual Gene

• The functional unit of heredity.

• It is found at particular locations on the chromosome, and the nucleotide sequence determines the primary structure of all polypeptides and proteins.

Allele

• Variety of a gene

• An alternative form of a gene that occurs at the same location on homologous chromosomes.

• For example, the gene that codes for the ABO blood type has three alleles, A, B, and O.

Dominant Allele

• The allele expressed in a phenotype when a genotype is heterozygous.

• Represented by Upper-case letters

• For example, in human blood groups the A and B alleles are dominant to the O allele.

• A person with an AO or BO genotype has type A or B blood, respectively.

Recessive Allele

• The allele not expressed in the phenotype when the genotype is heterozygous.

• Are only expressed when the genotype is homozygous.

• Represented by Lower-case letters

• For example, in human blood groups the O allele is recessive.

• It is only expressed when the genotype is OO, and not expressed when the genotypes are AO or BO.

Homozygous

• A genotype with the same alleles for a gene on

• Ex: 2 brown eye alleles

• For example, in human blood groups this conditions would be: genotypes AA, BB, and OO.

Heterozygous

• A genotype with different alleles for a gene on homologous chromosomes.

• Ex: having brown and blue eye allele

• For example, in human blood groups the heterozygous conditions would be: genotypes AB, AO, and BO.

Genotype

• The actual alleles present for a given gene on homologous chromosomes.

• What its made of

• Genotypes are expressed in letters.

• For example, human blood group genotypes are, AA, AB, AO, BB, BO, and OO.

Phenotype

• The outward expression of a genotype, or what the trait looks like externally, or physically.

• For example, in human blood groups the phenotypes determine whether a person has type A, B, AB, or type O blood.

• Are as described adjective words.

Codominant

• A genetic condition that occurs when neither allele is dominant over another, but both are expressed in the phenotype.

• For example in human blood groups A and B alleles are codominant, and a person with the AB genotype has type AB blood as the phenotype.

Homozygous Dominant

BB

Homozygous Recessive

bb

*In order for a person to outwardly display a recessive trait, they must be this

Heterozygous

Bb

Punnett Squares

is a 2x2 or 4×4contingency table with the possible gametes for each parent on its axes.

Monohybrid Cross

• Inheritance of one characteristic

• 2×2

• punnette squares

Dihybrid Cross

• Inheritance of two characteristics

• 4×4

• punnette squares

Homozygous Dominant or Heterozygous

If you are dominant for a character, your genotype could be either

Homozygous Recessive

If you are recessive for a character, your genotype must be

Phenotype: Brown Eyes

Phenotype Ratio: 3:1

75% chance for brown eyes

In humans brown eyes are dominant over blue eyes. What are the phenotypes and phenotypic ratio of offspring if you crossed a heterozygous brown eyed person to a heterozygous brown eyed person?

Genotype: Ratio 2:2

Ww - II

ww - II

Phenotype: Ratio 2:2

Widow’s peak - II

Straight - II

50% chance of having a widow’s peak

A widow’s peak hairline is dominant to straight hairline. Cross a heterozygous widow’s peak hairline person to a straight hairline person. Give all genotypic and phenotypic results.

Possible Offspring Genotypes:

• TT - homozygous dominant

• Tt - heterozygous

All offsprings will be able to roll their tongue

In humans tongue rolling is dominant to non-tongue rolling. What would be the expected type of offspring if a homozygous tongue roller was crossed to a heterozygous tongue roller? 

Genotype of parents

• TT - homozygous dominant

• tt - homozygous recessive

A Tall plant (T) is dominant to short plant (t). If all offspring of a cross are heterozygous, what were the genotypes of the parents?

Work:

Bob: T_(has to be t) Mother: tt Sally: tt

After using a punnett square:

50% their child will NOT be able to roll tongue

In humans, tongue rolling is a dominant trait, those with the recessive condition cannot roll their tongues. Bob can roll his tongue, but his mother could not. He is married to Sally, who cannot roll her tongue. What is the probability that their first born child will not be able to roll his tongue? 

Work:

Parent 1: BB (eye) rr (hair)

Parent 2: BB (eye) Rr (hair)

After using punnett square:

Child will have Brown eyes and a 50% chance of Red hair.

A couple has three children, all of whom have brown eyes and blond hair. Both parents are homozygous for brown eyes (BB), and one is blond (rr) while the other is a redhead (Rr). What is the probability that the next child will be a brown-eyed redhead?

Work: BbTt x BBtt

Punnett square: Bb BB

- both dominant so will have black fur

Punnett square: Tt tt

- one is dominant and the other is recessive, using the chart, it will have a 50% of either a long or tail

Black fur in mice (B) is dominant to brown fur (b). Short tails (T) is dominant to long tails (t). What proportion of the progeny of the cross BbTt x BBtt will have black fur and long tails?