MBG MIDTERM NOTES - INDEPTH and BASED OFF PREVIOUS EXAMS

Lecture 1 – Review of Basic Principles of Heredity and Pedigree Analysis

1. A gene is the fundamental unit of heredity
- Section of DNA that encodes for a certain trait
2. Genes come in multiple forms called alleles
- One or two alternative forms of a gene
3. Genotype determines phenotype
- Genotype: set of alleles possessed by an organism
- Phenotype: detectable expression of the genotype
4. Genetic information is carried in DNA
5. Genes are located on chromosomes
6. Genetic information is transferred from DNA to RNA to protein

Heterozygote: organism possessing two different alleles at a locus
Homozygote: organism possessing two of the same alleles at a locus

Monohybrid Cross (one trait vs one trait) Gregor Mendel
 Pea experiment

- P generation
1. Crossed a plant homozygous for round seeds (RR) with plant homozygous for
wrinkled seeds (rr)
2. Two alleles in each plant separated when gametes (reproductive cell) formed, one
allele went into each gamete

- F1 generation
1. Gametes fused to product heterozygous F1 plant that had round seeds because
round is dominant over wrinkled
2. Mendel self-fertilized F1 to produce F2

- F2 generation
1. Outcomes when breeding two heterozygous plants (Rr) produced a 3:1 ratio of
heterozygous and homozygous plants
o ¼ RR
o ½ Rr
o ¼ rr

Dihybrid Crosses (two traits vs two traits)
E.g. yellow and round seed (GGWW) vs green and wrinkled seed (ggww)

- P generation
1. Each parent homozygous seed produced a gamete
o Monohybrid cross between GG x WW and gg x ww producing two gametes GW
and gw

, o Since gametes only contain ONE allele from each parent

- F1 generation
1. Monohybrid cross between the two gametes GW x gw produces 4 kinds of gametes
in equal portions
o ¼ GW
o ¼ Gw
o ¼ gW
o ¼ gw
2. Self-fertilization of F1 generation using a dihybrid cross produces F2 generation

- F2 generation
1. Crossing GW Gw gW gw X GW Gw gW gw produces a 9:3:3:1 ratio




Mendel’s 1st Law of Segregation
1. Each individual organism possesses two alleles encoding a trait (Rr or rr or RR)
2. Alleles separate when gametes are formed (RR  R and R, Rr  R and r)
3. Alleles separate in equal proportions

Mendel’s 2nd Law of Independent Assortment
1. Alleles at different loci separate independently
- The outcome of one gene isn’t related to other genes

Pedigree

,Recognizing Patterns of Inheritance

Autosomal Recessive Trait: mutation that needs two recessive alleles to be affected
 Appears in both sexes with equal appearance
 Tends to skip a generation
 Affected offspring are usually born from unaffected heterozygous carrier parents
 If parents are both heterozygous you can assume one child is affected

Autosomal Dominant Trait: mutation that only needs one dominant allele to be affected
 Appears in both sexes with equal appearance
 Both sexes transmit the trait to their offspring
 Does not skip generations
 Affected offspring must have an affected parent unless they possess a new mutation
 When one parent is affected (heterozygous) and the other is unaffected (homozygous
recessive) half of the offspring will be affected
 Unaffected parents do not transmit the trait




X-linked Recessive Trait: genetic conditions associated with mutations on the X chromosome
where males need one X recessive allele, but females need both X recessive alleles
o Males = XY
o Females = XX

,  Usually more males are affected than females because males only carry one X
chromosome
 Affected sons are usually born to unaffected mothers
 Skips generations
 Half of a mother carrier (heterozygous) sons are affected
 Never passed from father to son because father gives son the Y chromosome
 All daughters of affected fathers are carries because they get one X chromosome from
their father
X-linked Dominant Trait: genetic conditions associated with mutations on the X
chromosome where both X alleles need to be dominant, males need one dominant X and
females need two dominant X alleles
 Both males and females are usually affected but more often females are because they
always get one X allele from their father
 Does not skip generations – affected sons must have an affected mother and affected
daughters must have either an affected mother or father
 Affected fathers pass the trait to all their daughters and they will be infected
 Affected mothers (if heterozygous) pass the trait to half their sons and half to their
daughters




Recessive Mutations: often involves loss of gene function:
 Null/Amorphic Alleles
1. A non-functional protein is produced
2. No protein is produced
 Hypomoprhic Alleles
3. A poorly functioning protein is produced
4. Reduced amounts of a normally function protein is produced

Tricks for Pedigrees
- Always assume if trait is rare and autosomal recessive (aa) then people who marry into
the pedigree are homozygous normal dominant allele (AA) unless question states