BIOL2110 - Week 15 - Meiosis and Genetics - Notes

Exam 4

• Exam 4 on 5/12.
• Lab manuals due 5/12.
• Reading assignments (Ch 8 and Ch 9) due 5/13.

Meiosis Concept Check

• Why is sexual reproduction advantageous over asexual reproduction?
• What is the main purpose of meiosis?
• How many daughter cells are produced at the end of meiosis, and are they haploid or diploid?
• What is the difference between meiosis I and meiosis II?
• Why is genetic variation important, and how does meiosis contribute to it?
• Name the stages of meiosis in the correct order.
• What happens to homologous chromosomes during anaphase I?
• What is synapsis and recombination, and during which phase does it occur?
• How does independent assortment contribute to genetic diversity?
• How does nondisjunction affect the gametes' chromosome numbers?

Genetics: Simple Inheritance

• Genes are located on chromosomes, which are physically transmitted from parent to offspring.
• Diploid organisms have two copies of each chromosome; one chromosome from each parent (homologous chromosomes) is passed to offspring.
• Alleles are alternative versions of the same gene that have different nucleotide sequences.
• Different alleles arise from variation in genes.

Genes Affect Traits

• Genotype: The particular genetic or allele makeup of an individual.
• Phenotype: The observable or measurable features of an individual.

Patterns of Inheritance: Complete Dominance

• Dominant alleles mask the presence of a recessive allele.
• Dominant alleles are expressed in phenotype, even when only one copy is present. Designated by uppercase letters. Example: $(B)$.
• Recessive alleles are masked by dominant alleles.
• Recessive alleles are expressed in phenotype in the absence of a dominant allele. Designated by lowercase letters. Example: $(b)$.

Homozygote and Heterozygote

• Homozygote: Two identical alleles.
  • Homozygous dominant $(AA)$: phenotype is dominant.
  • Homozygous recessive $(aa)$: phenotype is recessive.
• Heterozygote: Two different alleles $(Aa)$: phenotype is dominant.

Dog Fur Color Example

• Black fur = $(B)$ (dominant allele).
• Tan fur = $(b)$ (recessive allele).
• Male Dog:
  • From mom (maternal): Allele $(b)$.
  • From dad (paternal): Allele $(b)$.
• Female Dog:
  • From mom (maternal): Allele $(B)$.
  • From dad (paternal): Allele $(b)$.

Practice Example

• $B$: dominant (black fur).
• $b$: recessive (tan fur).
• Female dog:
  • Genotype: Heterozygous or homozygous?
  • Phenotype:
• Male dog:
  • Genotype: Heterozygous or homozygous?
  • Phenotype:

Punnett Square

• A diagram to determine the probabilities of offspring having particular genotypes.
• Based on genotypes of the parents.
• Matches up all possible gametes.

Punnett Square Practice

• Male and female dogs reproduce: Make a Punnett square.
• What genotypes and phenotypes could their puppies have?
• What is the probability of each genotype?
• What is the probability of each phenotype?

Complex Inheritance

Incomplete Dominance

• Incomplete dominance is shown when one allele doesn’t completely mask the presence of another allele.
• There is a measurable effect on the phenotype of having one versus two copies of the dominant allele.

Codominance

• A form of inheritance in which the effects of both alleles are displayed in the phenotype of a heterozygote.
• Both maternal and paternal alleles contribute equally and separately to the phenotype.

Codominance: ABO Blood Type

• Three blood group alleles: A, B, and O.
• Possible genotypes are combinations of these three alleles: $(AA, AO, BB, BO, AB, OO)$.

ABO Blood Type Details

• Blood type alleles A and B are codominant.
• O is recessive to both A and B.
• Type A:
  • Markers: Type A.
  • Genotype: $(AA$ or $AO)$.
  • Can donate to: Type A or AB recipient.
  • Can receive from: Type A or O donor.
• Type B:
  • Markers: Type B.
  • Genotype: $(BB$ or $BO)$.
  • Can donate to: Type B or AB recipient.
  • Can receive from: Type B or O donor.
• Type AB:
  • Markers: Type A and B.
  • Genotype: $(AB)$.
  • Can donate to: Type AB recipient.
  • Can receive from: Type A, B, AB, or O donor.
• Type O:
  • Markers: Neither A nor B.
  • Genotype: $(OO)$.
  • Can donate to: Type A, B, AB, or O recipient.
  • Can receive from: Type O donor.