Meiosis
What is the end goal of Meiosis?
To create 4 cells which are all haploid for reproduction.
In a monohybrid cross between two heterozygous parents (Aa × Aa), what fraction of the offspring are expected to be homozygous dominant?
1/4 (25%) are homozygous dominant (AA) from Aa × Aa.
In a dihybrid cross tracking two independent traits (AaBb × AaBb), what phenotypic ratio is expected in the offspring (Mendelian independent assortment)?
9:3:3:1 phenotypic ratio (dominant A dominant B : dominant A recessive b : recessive a dominant B : recessive a recessive b).
Define "homologous chromosomes."
Homologous chromosomes: chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location.
What occurs during Anaphase I?
Homologous pairs are separated into opposite parts of the cell.
Define the terms "phenotype" and "genotype."
Phenotype = observable traits; Genotype = genetic makeup (allele combination).
What is the first step in a dihybrid cross?
Create "mini-punnett square" and find the possible allele combinations for each parent.
What is a gamete?
Gamete: a sex cell (sperm or egg) that is haploid and can fuse during fertilization.
What occurs during Metaphase II?
Chromosomes align in the center of the cell.
If A = tall (dominant) and a = short (recessive), what phenotypic ratio do you expect from a cross of Aa × aa?
Cross Aa × aa yields 1/2 (50%) heterozygous (Aa) and 1/2 (50%) homozygous recessive (aa) — phenotypic ratio: 1 tall : 1 short.
Set up the gametes for a parent with genotype AaBb. List all unique gametes.
Gametes from AaBb: AB, Ab, aB, ab.
Define "haploid" and "diploid" and give the symbol commonly used for each (e.g., n or 2n).
Haploid = one set of chromosomes, symbol nn. Diploid = two sets of chromosomes, symbol 2n2n.
During what phase does crossover occur?
Prophase I
What would a cross between homozygous recessive and heterozygous produce?
How many genotypes result from two homozygous dominant, homozygous dominant parents?
1
What is "crossing over" and during which substage of prophase I does it typically occur?
Crossing over = exchange of chromosome segments between homologous chromosomes; occurs during pachytene (prophase I substage).
What is the purpose of crossover?
To increase randomization
A plant with purple flowers (dominant P) is crossed with a white-flowered plant and all offspring are purple. What can you infer about the genotype of the purple parent? Explain.
What is Purple parent is likely homozygous dominant (PP) if all offspring with white (pp) parent are purple — inference: purple parent is PP (if white parent is pp) because all offspring show dominant phenotype.
A dihybrid cross between AaBb and Aabb is performed. Use a Punnett-square reasoning (briefly) to state the expected phenotypic ratio for trait A (dominant vs. recessive) among offspring.
Cross AaBb × Aabb effectively for trait A is Aa × Aa? More directly: half the gametes from AaBb carry A and half a; Aabb gametes carry A or a with b fixed. Expected phenotypic ratio for trait A among offspring: 1 A_ : 1 aa (i.e., 1 dominant : 1 recessive) — so 1:1.
Define "allele," "dominant," "recessive," and "genetic locus" in one concise sentence each.
Allele = one of two or more versions of a gene; Dominant = allele expressed when present (masks recessive); Recessive = allele expressed only when two copies present; Genetic locus = specific location on a chromosome where a gene resides.