MEIOSIS: Genetic Variation / Mistakes in Meiosis (Sections 11-3,11-4;)
RECALL: Mitosis and Meiosis differ in several key ways: MITOSIS: MEIOSIS: 1 round of cell division 2 rounds of cell division Produces 2 DIPLOID daughter cells Daughter cells are identical to parent cell Produces body cells Produces 4 HAPLOID daughter cells Daughter cells are different from parent cell AND from each other Produces GAMETES (sperm and egg cells)
How does MEIOSIS lead to GENETIC VARIATION? 1) RANDOM ASSORTMENT: chromosome shuffling ; how the chromosomes line up in metaphase I
CHROMOSOME SHUFFLING: Example: in pea plants, there are 7 pairs of chromosomes; each of the 7 pairs can line up in 2 different ways this means that there are 128 different sperm or egg cells possible! (2 7 = 128) **the lining up of homologous pairs of chromosomes during metaphase I is a RANDOM PROCESS!
CHROMOSOME SHUFFLING: And since there are 128 different possible sperm cells, and 128 different possible egg cells, the # of possible combinations is: 128 x 128 = 16,384!
CHROMOSOME SHUFFLING: **So how many combinations are possible in humans??? Answer: 2 23 = 8,388,608 different egg or sperm cells and
8,388,608 x 8,388,608 = Over 70 trillion different zygotes possible!
How does MEIOSIS lead to GENETIC VARIATION? 2) CROSSING OVER: -occurs in prophase I when chromosomes line up with their homologous partner -chromosome segments are exchanged between homologous chromosomes -produces brand new combinations of genes on chromosomes
CROSSING OVER: -typically, there are 2 or 3 crossovers per chromosome pair -this means an almost endless # of -this means an almost endless # of different possible chromosomes!
Gametes
GENETIC RECOMBINATION: this reassortment of chromosomes and genetic information as a result of: -independent segregation ( shuffling ) -crossing over a major source of variation among organisms; the raw material that forms the basis for evolution (natural selection!)
MISTAKES IN MEIOSIS: sometimes an accident occurs and the chromosomes fail to separate correctly this is called NONDISJUCTION
NONDISJUCTION recall: during meiosis I, one chromosome from each homologous pair moves to each pole of the cell BUT, occasionally, both chromosomes of a homologous pair move to the SAME pole the result: 2 types of gametes: one with an extra chromosome, one missing a chromosome
TRISOMY: a condition in which the zygote (the cell that is formed when a sperm and egg cell unite) has an extra copy of a chromosome EXAMPLES: trisomy 21 (= Down Syndrome) XXY (= Klinefelter Syndrome)
MONOSOMY: occurs when a gamete with a missing chromosome fuses with a normal gamete result is a zygote lacking a chromosome (only 1 copy = monosomy) most zygotes with monosomy do not survive EXAMPLE of a non-lethal MONOSOMY: 1 X chromosome: Turner Syndrome
11-3 Heredity Following Different Rules Vocabulary: Karyotype Incomplete Dominance Sex chromosome Sex-linked traits Autosome Polygenic inheritance Key Concept: How is sex determined? How do small changes in DNA cause genetic disorders?
Human Chromosomes To examine human chromosomes, biologist photograph cells in mitosis when the chromosomes are fully condensed Cut out the chromosomes from the picture and group them together in pairs This type of picture is called a KARYOTYPE
Karyotypes A karyotype can tell you THREE things: 1. Sex (male or female) 2. Irregular numbers of chromosomes 3. Any mutations in the chromosomes Basically: all the chromosomes in a cell are displayed and can be examined for any abnormalities
Remember Humans have 46 chromosomes (23 pair) 2 of them are sex chromosomes they determine what sex you are XX = female XY = male 44 of them are autosomes they do not determine what sex you are
Who determines the sex of a child? The mother or the father? THE FATHER!!!!
Why does the father determine the sex of the offspring??? Mom is XX, she can donate either one X chromosome or the other X chromosome Dad is XY, he can donate either an X chromosome or a Y chromosomes. If the offspring receives the father s X, it is female If the offspring receives the father s Y, it is male
The Y chromosome If a Y chromosome is present, the person is male X chromosomes contain genes necessary for growth/development No cases of a person born with being 45,0Y Probably spontaneously aborted (miscarriage)
Many Patterns to Inheritance Incomplete dominance Codominance Sex-linked traits Polygenic traits Multiple alleles
What makes an allele dominant or recessive? Gene=sequence of DNA that codes for a protein Usually: Dominant alleles code for the correct production of that protein Recessive alleles code for no/wrong protein production Heterozygous condition the normal (dominant) allele will still cause correct protein production
We know that dominant traits usually mask recessive traits What happens when this doesn t happen? We get incomplete dominance
Incomplete Dominance Pattern where the phenotype of the heterozygous is intermediate between the two homozygous
For Complete Dominance: YY = yellow yy = white Yy = yellow For Incomplete Dominance RR = red rr = white Rr = pink The flower has blended both traits
The situation in which a heterozygote shows the phenotypic effects of both alleles fully & equally Codominance
Codominance For example: BB= brown bb = white Bb = brown and white The cow has both brown and white hairs