SEXUAL REPRODUCTION & GENETICS – CHP10

10.1 – Meiosis10.2 – Mendelian Genetics10.3 – Gene Linkage

Gregor Johann Mendel (1822– 1884) was a priest and scientist, and is often called the father of genetics for his study of the inheritance of traits in pea plants

10.1 MEIOSISObjectives1.Explain the reduction in chromosome number that occurs in Meiosis2.Recognize and summarize the stages of Meiosis3.Analyze the importance of meiosis in genetic variation

VOCAB•Gene•Homologous chromosome•Gamete•Haploid•Fertilization•Diploid•Meiosis•Crossing over

Meiosis Summary - division of gametes (sex cells) - 2 nuclear divisions (meiosis I and meiosis II) - results in FOUR cells from the parent

HAPLOID – cell with half the number of chromosomes (n) as a diploid (2n)DIPLOID – cell with two copies of each chromosome

Somatic (body cells) are diploid (2n) and gametes (sex cells) are haploid (n)If n = # of chromosomesHUMAN sex cells = n = 23Somatic cells = 2n – 46

Fertilization - oocyte + sperm results in a diploid zygote

Importance of Meiosis - it results in GENETIC

VARIATION

HOW?

- Pairs of homologous chromosomes line up at the equator during METAPHASE I.- Depending on how the chromosomes line up, FOUR gametes with FOUR different combinations of chromosomes can result.-Genetic variation also is produced by crossing over (PROPHASE I) and during fertilization when gametes randomly combine.

Mendelian GENETICS – 10.2Objectives1.Explain the significance of Mendel’s experiments to the study of genetics2.Summarize the law of segregation and law of independence assortment3.Predict the possible offspring from a cross using a Punnett square

VOCABULARY - allele - dominant - recessive - homozygous - heterozygous - genotype - phenotype - hybrid

Q. Predict the possible offspring from a cross using a Punnett square – VOCABULARY

P1 – parental generationF1 or F2 – filial (offspring) generationAllele – alternative form of a single geneHomozygous – same alleles for single traitHeterozygous – different alleles for single traitGenotype – organism’s GENes(alleles)Phenotype – organism’s Physical/observable

characteristicsHybrid - heterozygousPurebred – homozygousTest cross – involves breeding of an organism that has the unknown genotype with on that is homozygous recessive for the desired trait

Gene Linkage – occurs when a particular genetic allele for genes are inherited jointly. They tend to stay together during meiosis and are genetically linked.

While homologous pairs of chromosomes are independently assorted in meiosis, the genes that they contain are also independently assorted only if they are part of different chromosomes.  Genes in the same chromosome are passed on together as a unit.  Such genes are said to be linked.   For example, the "A" and "B" alleles (in the illustration below) will both be passed on together if the lower chromosome is inherited.  "A" and "B" are linked due to their occurrence in the same chromosome.  Similarly, "a" and "b" are linked in the other chromosome.

Linked genes most likely account for such phenomena as red hair being strongly associated with light complexioned skin among humans.  If you inherit one of these traits, you will most likely inherit the other.

Sex-linkage – When a trait is carried on the X or Y chromosomes, it is called a sex-linked trait - Don’t confuse this with linked genes = when 2 genes are on the same chromosome

Out of your 23 pairs of chromosomes22 pairs = autosomes1 pair = sex chromosome (XX or XY)

XY XX

X Y X

XX XY

- You have to include the X and Y chromosomes in the Punnett Square

- Superscripts on the X and Y denote which allele is present

- Gene that controls this is on the X chromosome

Who is more likely to be color blind– men or women?

Men: only 1 X chromosome – if they have the recessive allele they don’t have another X to make up for it.

Red-Green Colorblindness

EXAMPLE: color blindnessXX – mask the recessive train as there are 2 XXY – because there is only 1 X, they are affected by the recessive trait

EXAMPLE: Hemophilia – recessive sex-linked

- The effects of this sex-linked, X chromosome disorder are manifested almost entirely in males, although the gene for the disorder is inherited from the mother.

- Females have two X chromosomes while males have only one, lacking a 'back up' copy for the defective gene.

- Females are therefore almost exclusively carriers of the disorder, and may have inherited it from either their mother or father.

QUEEN VICTORIA’S PEDIGREE