MEIOSISSEXUAL REPRODUCTION

• A combination of the genetic material of two separate individuals

• Humans have 46 chromosomes - 23 pairs• Humans reproduce by joining sperm and

egg from two parents• If sperm and egg each had 46

chromosomes, the baby would have 92• Therefore----- meiosis is the production of

gametes (sperm or eggs) with only 23

• Meiosis has the same stages as mitosis, but repeated twice each – end result- four haploid cells (23 chromosomes each). 46 chromosomes = diploid cell.

• Meiosis compared to mitosis

• All chromosomes in a cell have numbers – 1-22 pairs

• Sex chromosomes make the total 23

• They all come in pairs, paternal and maternal

• Therefore, you received one #1 chromosome from your Mom and the other from your Dad.

• Each cell MUST have one #1 chromosome and one #2 chromosome and so on. No more or less than 1.

• Therefore, during meiosis metaphase 1, homologous pairs match up. The homologous chromosomes are separated at anaphase 1.

• Each resulting sperm or egg can have paternal OR maternal #1 chromosome, paternal or maternal chromosome #2, etc.

• This is known as independent assortment.

• While the homologous pairs are matched in metaphase 1, they can trade pieces of DNA = crossing over

• This creates lots of genetic variation (good for the human species)

• Crossing over only occurs in meiosis

Nondisjunction

NONDISJUCTION – change in chromosome #

• Error in Meiosis• Homologous chromosomes fail to separate• Leads to abnormal chromosome # (aneuploidy)

– Down syndrome: trisomy 21• Characteristic facial features• Heart defects, mental retardation• 1/700 births• Incidence rises with mother’s age

– Non disjunction of sex chromosomes• Turner’s syndrome, only one X chromosome

• Female, short stature, sexually immature

– Klinefelter’s syndrome, XXY• Male, testosterone deficient, sterile

Mendelian genetics

• Genotype (genes), Phenotype, and alleles of a specific gene

• Capital letters for dominate alleles• Lowercase letters for recessive alleles• Every person has two alleles for each trait• Gametes have only one allele for each

trait• Genotype of recessive phenotype is

known (rr)

• Genotype of dominant phenotype: two possible – 1. homozygous dominant (RR), 2. Heterozygous (Rr)

• Test-cross determines genotype of dominant phenotype – RR X Rr – Punnett square

• Humans not test-crossed, but can look at ancestors

• Cystic fibrosis: recessive, absence of protein causes chloride ion channels to malfunction – CC, Cc, cc

• Carrier: unaffected, but can pass recessive allele to offspring

• Do example genetics problems on handout

• Blood types – multiple alleles – A,O,B• Sex linked genes – on X&Y chromosomes• Male (Y) chromosome all alleles

expressed• Colorblindness example: carrier mother

gives defective allele to half her sons and half her daughters

Amniocentesis

• Karyotype shows any chromosomal abnormalities