Genetics Flashcards
Genotype
Genes responsible for a trait. A gene produces enzymes that catalyze a reaction to produce a trait.
Phenotype
Expressed traits.
Monohybrid
Only studying one trait.
F1
First generation of offspring
Allele
Version of a gene.
Dominant gene
- For Mendelian traits, a dominant allele is expressed over a recessive allele.
- A recessive phenotype only has one genotype.
Homozygous
The alleles agree on a trait (BB or bb)
Heterozygous
Alleles don’t agree on a trait, so the dominant one is usually expressed (Bb).
Codominance
Both traits are completely expressed, there is no dominant gene (blood type AB).
Incomplete dominance
“In between” phenotype. The traits are combined. For example, a red flower and a white flower produce a pink flower.
How are genotypes and phenotypes predicted?
A Punnett square predicts the probability of genetic combinations, usually 4 possibilities.
Epistasis
The presence of a certain gene determines if another trait is expressed at all.
Autosomes
Non sex chromosomes. Only one pair of chromosomes determines sex.
What does each allele in a parent represent?
A gamete.
3 laws of Mendelian Genetics
- Law of Dominance
- Law of Segregation
- Law of Independent Assortment
Law of Dominance
For a monohybrid cross, organisms with contrasting traits are studied. One parent is homozygous for dominant alleles, the other for recessive. All offspring are heterozygous for the F1 generation, but express the dominant trait. The traits don’t combine.
Law of Segregation
- When offspring reproduce among themselves, a recessive phenotype appears.
- Alleles can segregate and recombine
- If two heterozygous offspring reproduce, the phenotype is 3(B):1(b). The genotypes are 1(BB):2(Bb):1(bb).
Law of Independent Assortment
- If 2 traits are considered, 4 alleles are considered, and combine to make 4 gametes
- Traits can segregate and combine independently, without being affected by other genes.
Dihybrid Cross
- Combines different alleles (8) for 2 traits to produce 16 possible combinations.
- Best way to solve is to find the probability for each trait independently and then multiply them.
Test Cross
- Uses a recessive organism to determine the genotype of another organism if the genotype is unknown
- If a recessive organism appears, the original organism was heterozygous
Sex linked traits
A trait whose allele is carried on one of the sex chromosomes. Most are X linked.
3 common X linked recessive traits
Hemophilia, color blindness, and male pattern blindness.
How can a female express an X linked trait?
Can only express the trait if she is homozygous for the X chromosome that carries the allele (it appears on both X chromosomes).
What is a carrier?
A person who carries the allele but does not express the trait. A female is a carrier if she is heterozygous for an X linked recessive trait. She can pass the allele to her offspring.
How can a male express an X linked trait?
A male will express the trait if he carries it on his X chromosome. Although the trait is recessive, a male doesn’t have another X chromosome to cancel out the gene’s effects.
Will the offspring of a male who expresses an X linked trait also express the trait if their mother is normal?
- An X linked trait can’t pass from father to son because a father gives his son the Y chromosome. The male’s son will be normal.
- A father gives his daughter another X chromosome. The daughter will be a carrier, but if her mother is normal she will not express the trait.
Pedigree
- A chart that shows the presence of a phenotype in a given family
- Males are represented by squares, females by circles
- Horizontal line represents mating, vertical shows offspring
- Individuals with that phenotype are shaded.
How to determine if a condition is dominant or recessive (pc)
Recessive conditions skip generations, so if neither parents nor offspring of an individual display the condition, it’s recessive.
How to determine if a condition is sex linked or autosomal (pc)
Sex linked conditions are more common in males, so the condition is probably sex linked if many more males are affected than females.
How to determine if an individual is homo or heterozygous for a condition (pc)
- If an individual is female but not affected by a sex linked condition, she can’t be homozygous.
- If a father has the condition but his daughter doesn’t, the daughter doesn’t, the daughter is heterozygous
- All daughters of males affected by a sex linked condition are carriers.
How to determine the possibility of producing an affected son/daughter (pc)
- If the condition is autosomal, determine if it is dominant or recessive, then use a Punnett square with regular alleles. Sex doesn’t matter.
- For a sex linked condition, make a Punnett square using chromosomes and the condition. Remember the probability of an affected father and a normal woman producing an affected son is 0%.
