Unit 5: Meiosis and Genetics Flashcards
Diploid
A cell that has two complete sets of chromosomes (one from each parent)
Somatic cells (skin cells, muscle cells). Mitosis. One division. 46 chromosomes
Haploid
A cell that has only one complete set of chromosomes. Gametes (sperm and egg cells). Meiosis. Two divisions (meiosis I and meiosis II). 23 chromosomes
Homologous chromosomes
Homologous chromosomes are pairs of chromosomes in a diploid organism that have the same structure, size, and genetic content but may carry different versions (alleles) of a gene. One chromosome in the pair is inherited from the mother, and the other from the father. These chromosomes align with each other during meiosis, which is important for the proper segregation of genetic material during cell division. Homologous chromosomes exist in diploid cells (before meiosis) but are separated during meiosis I. Gametes (sperm/egg) do not have homologous pairs, but when fertilization occurs, homologous chromosomes recombine in the new diploid zygote.
Summary of Meiosis I
Meiosis I:
Prophase I: Chromosomes condense, and homologous chromosomes pair up (synapsis), forming tetrads. Crossing over occurs, where homologous chromosomes exchange genetic material, leading to genetic variation. The nuclear membrane breaks down, and spindle fibers form.
Metaphase I: Tetrads align at the metaphase plate. Each homologous chromosome is attached to spindle fibers from opposite poles.
Anaphase I: Homologous chromosomes are pulled to opposite poles. The centromeres do not split yet, so each chromosome still consists of two sister chromatids.
Telophase I: Chromosomes reach the poles, and the nuclear membrane reforms around each set of chromosomes. Cytokinesis occurs, splitting the cytoplasm into two cells. These cells are haploid, meaning they have half the number of chromosomes.
Summary of Meiosis II
Prophase II: Chromosomes condense, and spindle fibers form in each of the two haploid cells. The nuclear membrane dissolves.
Metaphase II: Chromosomes align at the metaphase plate in both haploid cells.
Anaphase II: The sister chromatids are pulled apart toward opposite poles.
Telophase II: Chromatids reach the poles, and nuclear membranes form around each set of chromosomes.
Cytokinesis occurs AFTER meiosis II, resulting in four non-identical haploid gametes.
Errors during cross over
Inversion: A segment is reversed, which can disrupt gene function or cause alignment issues during meiosis.
Deletion: A segment of DNA is lost, causing the loss of genes, which can disrupt normal function or development.
Duplication: A segment is copied, leading to extra genes that may cause overexpression and developmental issues.
Translocation: A segment of one chromosome attaches to another, which can disrupt gene function and potentially cause diseases or infertility.
Nondisjunction
the failure of chromosomes or sister chromatids to separate properly during cell division (meiosis or mitosis). This results in one daughter cell having an extra chromosome and the other having one less chromosome, leading to genetic disorders such as Down syndrome, Turner syndrome, or Klinefelter syndrome.
How do meiosis, independent assortment of chromosomes and fertilization create genetic diversity
Meiosis reduces the chromosome number by half, producing haploid gametes that carry a unique set of genetic information from each parent. During meiosis I, independent assortment randomly arranges and separates homologous chromosomes, leading to various combinations of chromosomes in the gametes. Finally, fertilization combines the genetic material from two individuals, further increasing genetic diversity. Together, these processes ensure that offspring inherit a unique combination of genes, promoting variation within a population.
Alleles
Alternative versions of genes that account for variations in inherited characters
Each gene resides at a specific locus on a specific chromosome
Organisms inherit two alleles, one from each parent
Phenotype
observable characteristics encoded by an organism’s genotype (inherited genes)
Genotype
the genes/alleles an organism has
Testcross
breeding a mystery individual with a homozygous recessive individual. If any offspring display the recessive phenotype, the mystery parent must be heterozygous
Law of Segregation
when an individual produces gametes, the two copies of a gene separate so each gamete receives only one copy
Law of Independent Assortment
each pair of alleles segregates independently of each other pair of alleles during gamete formation
Predict inheritance with probability
If A and B are mutually exclusive then P(A or B) = P(A) + P(B)
If A and B are independent then P(A and B) = P(A) + P(B)
Ex:
What is the probability that the offspring will get a B allele from the male beetle?
4/4 100%
What is the probability that the offspring will get a B allele from the female beetle?
2/4 50%
Calculate the probability that the offspring will have the genotype BB from this cross. Check your answer with the Punnett Square to the right
4/4 x 2/4 = 8/16 = 1/2 (50%)
How to do dihybrid cross
Do a punnett square for each gene from each parent
Then if asked the probability of a specific combination just multiply the probability from each punnett square
Complete dominance
when phenotypes of the heterozygote and dominant homozygote are identical
Chromosomal Theory of Inheritance
Mendelian genes have specific loci (positions) on chromosomes. Chromosomes undergo segregation and independent assortment. The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment
Incomplete dominance
Non-Mendelian, as it results in a blend of traits rather than a clear dominant-recessive expression.
Codominance
Non-Mendelian, as both alleles are fully expressed rather than one being dominant over the other
Multiple Alleles
Non-Mendelian. The four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
Epistasis
A gene at one locus alters the phenotypic expression of a gene at a second locus. Non-Mendelian.
Ex: in Labrador retrievers and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy. Non-Mendelian.
Ex: pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
Polygenic Inheritance
where multiple independent genes have an additive or similar effect on a single quantitative trait. Vary in the population along a continuum. Best example is skin color. Non-Mendelian.
Sex-linked (X-linked) genes
X chromosomes have genes for many characters unrelated to sex, whereas the Y chromosome mainly encodes genes related to sex determination (such as SRY gene). X-linked genes follow specific patterns of inheritance. For a recessive X-linked trait to be expressed. A female needs two copies of the allele (homozygous). A male needs only one copy of the allele (hemizygous)
Linked Genes
Linked genes tend to be inherited together because they are located near each other on the same chromosome. Linked genes assort together during meiosis, which means they “violate” the law of independent assortment. Results of a cross with linked genes do not adhere to normal genotypic ratios!
Recombination Frequency
Genes on chromosomes switch places during crossing over which occurs during Prophase I of Meiosis. Crossover is random, but the likelihood that 2 genes crossover will increase if those genes are farther apart. The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency. Genes closer together are more likely to “stick together” and not switch places. These genes are said to be linked.
Recombination frequency formula
Recombination frequency = (# of recombinant offspring)/(total number of offspring) x 100
Chi squared
X^2 = (O-E)^2/E
Mendelian Ratios
- phenotypic ratio for a monohybrid cross (crossing individuals heterozygous for one trait) is 3:1 (dominant phenotype: recessive phenotype). the genotypic ratio is 1:2:1 (homozygous dominant: heterozygous: homozygous recessive)
- For a dihybrid cross (crossing individuals heterozygous for two traits), the phenotypic ratio is 9:3:3:1
