Meiosis and Chromosomal Inheritance

Chromosomes Karyotype – all chromosomes in an organism Homologous chromosomes – specific chromosome pair Gamete- sex cell, Ex: sperm and egg Somatic cells- all cells except for gametes

General Life Cycle Haploid- cell has 1 copy of each chromosome (n) What type of cells would have this? Diploid- cell has 2 copies of each chromosome (2n) What type of cells would have this? What does the (n) signify?

Mitosis vs. Meiosis

Goals of Meiosis Produce a cell that has half of the genetic material as somatic cells – Reduction Division – random separation of homologous chromosomes Increase variation in those sex cells produced – Segregation of Sister Chromatids – random separation of sister chromatids – Independent Assortment – Crossing over

Meiosis Stages

Meiosis I Book uses bivalents I use tetrads (usually makes more sense to my students)

Meiosis II

Meiosis – General Process Goal of Meiosis is to produce a cell that has half of the genetic material as somatic cells and to increase variation in those sex cells produced Reduction Division – random separation of homologous chromosomes Segregation of Sister Chromatids – random separation of sister chromatids

When Meiosis Goes Wrong What is the difference between these karyotypes?

Nondisjunction The failure of homologous chromosomes to separate properly during meiosis. Normal: Nondisjunction:

The results If the abnormal gametes is fertilized the results

For the Test You don’t need to know specifics about each disease. You will need to know what can happen when Meiosis does not occur correctly

Down Syndrome: Trisomy 21 1 in 900 live births Distinctive physical appearance Development: slow growth, developmental delays Health Impact; 40% have heart defects, Prone to respiratory infection, High leukemia rate.

Trisomy of chromosome 18: Edwards Syndrome (47, XX +18 or 47 XY +18) mental retardation small head, small eyes, small lower jaw congenital heart defects (90% of individuals) Infants have a 5% chance of surviving Occur in 1:8000 live births This baby only lived 5 days

Trisomy 13: Patau syndrome Occurs 1 in 6, 000 live births Congenital heart defects Mental retardation, severe Seizures Small head and eyes Scalp defects (absent skin) Cleft lip and/or palate Eyes close set -- eyes may actually fuse together into one Iris defects Ear Abnormalities Extra digits (polydactyly) Hernias Skeletal (limb) abnormalities

Monosomy The zygote is lacking a chromosome Organisms lacking one or more chromosomes rarely survive

Turner Syndrome Female (45 total chromosomes, 1 sex chromosome (X)) Only monosomy that is not lethal Don’t develop normal at time of puberty, underdeveloped breasts, rudimentary ovaries Infertile Treatment with hormone supplements can help these women lead normal lives

Klinefelter Syndrome Male (XXY) May have some learning difficulties Affected male may not develop normally at puberty In severe cases: rudimentary testes & prostate gland, sparse facial and pubic hair, long arms and legs, breast development may occur Infertility

Super Male Syndrome: XYY frequency is approximately 1 of 1000 males often are more physically active tendency to a delayed mental maturation an increased tendency for learning- problems in school, this means a need for early and adequate stimulation.

XXX syndrome (Trisomy X) 1 in 1,000 females Fertile Most are normal May have mild retardation

Goals of Meiosis Produce a cell that has half of the genetic material as somatic cells – Reduction Division – random separation of homologous chromosomes Increase variation in those sex cells produced – Segregation of Sister Chromatids – random separation of sister chromatids – Independent Assortment – Crossing over

Structures of Chromosomes Locus – location of a gene on a chromosome

Types of Chromosomes Autosomes = chromosomes 1-22 Sex chromosomes = X and/or Y  XX = female  XY = male Even though these chromosomes determine sex, only one gene on the Y chromosome actually determines sex

Law of Independent Assortment Chromosomes randomly distributed during meiosis Genes on different chromosomes inherited independently

Genetic Linkage Genes on same chromosome inherited together Law of Independent Assortment is increasing variation with the idea that a chromosome is a complete “package” which is true However we can take apart that package too!

Example: Fruit Flies Genetic cross of fruit flies with known genotypes Expected 1:1:1:1 ratio Observed 5:5:1:1 ratio Parental phenotypes more common than non-parental phenotypes

Cross-over Exchange of genes between homologous chromosomes Gametes possess unique combination of alleles Result: 4 different types of gametes

Example: Mendel’s Peas Flower color and seed color genes both located on chromosome 1 Distance between loci great enough that genes assort independently In other words they don’t seem to be linked because so much crossing over occurs Flower Color Seed Color

Example: Fruit Flies From this data, do you think that these genes are located closer or further apart on the chromosome? Explain. G W G W

Variation Independent assortment of chromosomes – 8,388,608 possible combinations with 23 different chromosomes Cross-over – think about crossing over in combination with the independent assortment of chromosomes… 8,388,608 combinations becomes infinitesimal! Fertilization- which sperm makes it!

Chromosomal Abnormalities Changes in chromosome number or structure – Deletions – Inversions – Translocations – Duplications

Example: Cri du chat Deletion of part of chromosome 5 Name based on the infant’s cry, which is high- pitched and sounds like a cat

Why Sex?