Meiosis and Genetic Inheritance Flashcards
Meiosis
Form of cell division that creates gametes that are suitable to be paired sexually with another gamete to contribute genetic information to the next generation
Germ cells undergo two rounds of nuclear division to produce four haploid daughter cells called gametes- unique genetic makeup
Process of Meiosis
Two rounds of division called meiosis I and meiosis II
Each consists of successive stages of prophase, metaphase, anaphase, and telophase
Meiosis I
Separate homologous chromosomes to produce two haploid cells, each with one copy of the 23 chromosomes
Known as reduction division
Prophase I
Homologous chromosomes line up alongside each other, matching genes exactly
Have four chromatids in homologous pair, called tetrads
Crossing over may occur, genetic recombination
Crossing Over or Genetic Recombination
Chromosomes “zip” along each other where nucleotides are exchanged forming the synaptonemal complex
- creates ‘X’ shape or chiasma under microscope
Gene Linkage
Where genes are physically located near one another, increasing probability that traits will be inherited together despite crossing over
Single Crossover
Chromosomes may exchange sections of genetic information just once
Double crossover
Chromosomes trade a segment once and then trade back a sub-section of segment
Each chromosome retains some of own original genetic material
Gene mapping
A technique which helps determine locations and relative distances of genes on chromosomes
Metaphase I
Two homologues remain attached and move to the metaphase plate
- Tetrads align along the plate with 23 tetrads in single file line in humans
Anaphase I
Homologous chromosomes each separate from their partner independently assorting to create two haploid cells
Telophase I
Nuclear membrane may reform, cytokinesis may occur
- these both occur in humans
New germ cells are haploid with 23 replicated chromosomes and are called secondary spermatocytes or secondary oocytes
Polar Body
Produced in female oocytes
One of the germ cells formed after telophase I becomes a polar body, has much less cytoplasm, and degenerates
Meiosis II
Proceeds through Prophase II, Metaphase II, Anaphase II, and Telophase II much like mitosis under light microscope
Final products are haploid gametes, each with 23 chromosomes
Four spermatocytes formed
One ovum is formed
Nondisjunction
During Anaphase I or II, if any chromosome does not split
Anaphase I: One cell has two extra chromatids and other is missing a chromosome
Anaphase II: Results in one cell having one extra chromatid and one cell lacking a chromatid
How does trisomy 21 happen?
Known as Down syndrome
Caused by nondisjunction of chromosome 21 in Anaphase I
Gametogenesis
Production of gametes
Different for males than for females
Gametogenesis in Males
Diploid progenitor cells called spermatogonia.
Spermatogonium undergoes mitosis to produce two diploid primary spermatocytes
Primary spermatocytes undergo meiosis I to become haploid secondary spermatocytes
Secondary spermatocytes undergo meiosis II to become spermatids
Spermatid loses cytoplasm and gains tail- sperm
Male gamete
Sperm
Gametogenesis in Females
Diploid progenitor cell is oogonium
Oogonium undergoes mitosis to produce two primary oocytes before female is born
At puberty, primary oocytes undergo meiosis I, making haploid secondary oocyte
Secondary oocyte arrested at metaphase II until penetrated by sperm (fertilization), complete meiosis II to form ootid, mature into ovum
Why does the female oocyte need to conserve cytoplasm by releasing polar bodies?
Once an ovum is fertilized, resulting zygote needs to undergo many rounds of division before it is able to implant on the uterine wall and establish a blood supply
Increased cytoplasm present in ovum provides nutrients necessary to sustain a zygote as it becomes a bastula and travels from fallopian tube to uterus
Genetic Leakage
Flow of genetic information from one species to another
Locus
Position on a chromosome
Wild type
Normal or most common allele type for a certain trait within a population
Genotype
Individual’s genetic makeup of a certain trait or allele
Phenotype
Expression of the trait of an allele
Expressed through the action of enzymes and other structural proteins which are encoded by genes
Complete dominance
Dominant allele masks expression of the recessive allele
Mendel’s pea plants, purple vs. white
Homozygous
Individual with a genotype having two dominant or two recessive alleles that are the same
Heterozygous
Individual with a genotype having one dominant and one recessive allele
AKA hybrid
Law of Segregation
Mendel’s first law of Heredity
Alleles segregate independently of each other when forming gametes during meiosis
Penetrance
Refers to the probability of a gene or allele being expressed if it is present
Penetrance of dominant allele is 100%
Expressivity
Measure of how much the genotype is expressed as a phenotype
Degree of expression
Relevant when considering incomplete dominance (intermediate phenotypes)
Incomplete dominance
When a heterozygous individual exhibits a phenotype that is intermediate between its homozygous counterparts
Co-dominance
Heterozygous individual exhibits both phenotypes
E.g. Human blood type alleles are co-dominant because a heterozygous individual exhibits A and B antigens
Punnett Square
Predicting genotypic ratios of offspring from parent genotypes
Law of Independent Assortment
Mendel’s Second Law of Heredity
Genes located on separate chromosomes assort independently of each other
For example, genes that code for distinct traits when located on separate chromosomes do not affect each other during gamete formation
Dihybrid Cross
Cross that can be demonstrated on a Punnett square for two distinct traits with a dominant and recessive allele for each in which we cross all possible heterozygous combinations with one another (WG, Wg, wG, wg) x (WG, Wg, wG, wg)
Phenotypic ratio: 9:3:3:1
Sex Chromosome
Each partner in the 23rd pair of chromosomes (X or Y)
Y chromosome contains only a few genes
Sex-linked trait
Genes located on the sex chromosomes (usually the X chromosome, because the Y chromosome has very few genes)
In males sex-linked genes on X chromosome are expressed whether dominant or recessive
Barr body
Most somatic cells have one of the X chromosomes in a female randomly condense to forma tiny dark object
Formed at random, so active allele is split approximately evenly among all cells
- recessive allele usually still only displayed in homozygous individual
Carrier
Female may have recessive trait on 23rd pair of chromosomes without expressing it
- strong chance of being expressed in male offspring
Hardy-Weinburg Equilibrium
Theoretical state of suspended evolution in which there is no net change happening in allelic frequencies over time
Hardy-Weinburg equation
p^2 + 2pq + q^2 = 1
Equation predicts genotype frequencies of a gene with only two alleles in a population in Hardy-Weinberg equilibrium
Difference between Mendel’s calculations and Hardy-Weinberg equation?
Rather than having a 1 in 2 chance of inheriting an allele (A or a) from one of the two parents, here the probability corresponds to the proportion of that allele (A or a) in entire population
Gametes
Haploid reproductive cells
