Question

MCAT Tutorial: Introduction to Genetics

Answer 1

\({\bf{Basic~Terminology:}}\)
- chromosomal theory of inheritance: chromosomes carry the units of inheritance (genes) at specific loci
- locus/loci: the location of a gene on a chromosome (singular/plural)
- law of segregation: for a particular gene, the alleles will separate during gamete formation
- law of independent assortment: genes for a particular trait are inherited independently of other genes
- phenotype: the physical traits of an organism
- genotype: the genes encoding a trait
- alleles: variations of a particular gene
- homozygous: containing two copies of the same allele
- heterozygous: containing two copies of different alleles

\({\bf{Punnet~Squares:}}\) are a tool used to measure the probabilities of genotype/phenotype inheritance among the offspring of a particular set of parents.
for a monohybrid cross (one gene, two alleles per parent) we will set up a 2x2 table like so:

Answer 2

for each cell bring the allele directly horizontal and the other allele which is directly vertical into the cell, like so:

Answer 3

notice how we have 1 FF, 2 Ff, and 1ff for the genotypes, giving us a 1:2:1 genotypic ratio

now, we will assume the capital letter F represents a dominant allele, thus masking the recessive allele in every genotype that contains at least one dominant allele

giving us 3 dominant phenotypes and 1 recessive phenotype, or a 3:1 phenotypic ratio for the F2 generation

This type of cross is called a monohybrid cross

For a dihybrid cross, assume the two genes are unlinked (not on the same chromosome pair) and thus exhibit independent assortment. Each gamete, as usual, contains one copy of each gene, as exhibited by this cross of peas:

Answer 4

looking at the genotyypes we have: 1 YYRR; 2 YYRr; 1 YYrr; 2 YyRR; 4 YyRr; 2 Yyrr; 1 yyRR;2 yyRr; 1 yyrr, or a 1:2:1:2:4:2:1:2:1

however, it is more pertinent to remember the genotype ratio 9:3:3:1

Other types of crosses:

Testcross: If we want to know the phenotype of unknown organism we will do a testcross with that organism + a homozygous recessive individual

Backcross: F1 hybrid + one of its parents, typically used to reintroduce an unusual trait back into the breed

\({\bf{Dominance~Patterns}}\)
the pattern studied above is complete dominance (where a dominant allele will completely mask a recessive allele. A heterozygous individual is a carrier of a recessive allele, this becomes very relevant in disease inheritance)

- incomplete dominance: a heterozygous individual expresses a third phenotype, which is a combination of the dominant and recessive phenotypes (ex. homozygous red and white flowers --> pink flowers). This is very rare in humans.
- codominance: heterozygous individual expresses a third allele that is not a blend of traits (ex. blood types. Three alleles A,B, and i where A and B are codominant with each other thus expressing the AB phenotype)

\({\bf{Penetrance~and~Expressivity:}}\)
- penetrance: the probability that a trait will be expressed given the gene is inherited (the expression of a gene can depend on epigenetic factors like environment
- expressivity: degree to which the allele is expressed

Answer 5

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT