i. Children not precise replicas of parents ii. Inheritance - had been a mystery generations a. Gregor Mendel - Monk in Brno Czech Republic

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1 1. Introduction a. Like begets like i. Children not precise replicas of parents ii. Inheritance - had been a mystery iii. Gene is unit of inheritance iv. Blending inheritance 1. People knew that offspring looked like a combination of both parents 2. Before Mendel they thought that the traits blended in offspring a. Like mixing white and red paint makes pink b. Didn't account for weird reappearances of traits in second generations 2. Mendel's investigations a. Gregor Mendel - Monk in Brno Czech Republic i. Observations based on garden peas - but generalizable to all organisms ii. Basis of science of genetics iii. Established a series of laws - published in 1866 iv. Observations ignored until they were rediscovered in the early 1900's 3. Chromosomal basis of inheritance a. Mendel's laws parallel the behavior of chromosomes in meiosis i. Gametes responsible for providing genetic information of offspring b. Meiosis i. Chromosomes come in pairs - homologous 1. Gametes have only 1 member of homologous pair a. Haploid number - N b. Diploid number - 2N 2. Alleles a. Alternative forms of genes ii. Meiosis separates homologous chromosomes 1

2 1. Each gamete gets one of homologous pair a. Human diploid number is 46 b. Each gamete gets 23 chromosomes c. Sex determination i. McClurg in 1902 found insects produce 2 kinds of sperm 1. On kind has a smaller chromosome 2. These produced female offspring ii. Sex determination 1. Different in different species 2. Mammals a. Male is the one with the smaller chromosome XY b. Females are XX 4. Mendelian Laws of Inheritance a. Mendel's first Law i. Law of segregation - in the formation of gametes, paired factors specifying alternative phenotypes segregate independently of one another 1. Visible characters are called phenotypes -tall or dwarf peas a. Cross tall and dwarf peas - all progeny are tall 2

3 To Make the F1 generation: TT What Gametes can be made? Female T x T tt Male t t Only F1 Hybrids are genotype All Tall plants Female T x t Male T t TT tt Special results 75% tall 25% short b. Cross F1 generation - get segregation of traits i. 25% short ii. 75% tall c. Repeat this for other traits get same statistical results d. Mendel called the tall factor "dominant" - the short factor was "recessive" - dominant factor masks the recessive i. Each individual has 2 "doses": of a trait ii. Can be homozygote = TT or tt iii. Can be heterozygote = e. There are 3 hereditary types in the peas TT, tt, i. These are called genotypes f. If Mendel selfed the F2 plants only the dwarfs bred true tt i. 1/4 of talls breed true and other talls do not breed true g. Factors controlling tallness and dwarfness are units that did not blend together i. These units we call genes ii. The versions tall and dwarf are called alleles 3

4 h. Punnett squares i. Way of showing genetics Gametes T T T TT tall tall T tall dwarf ii. i. Test cross i. When one allele is dominant, you can't determine genotype of individuals with dominant trait ii. You only know the genotype of individuals with the recessive trait iii. Cross individual with recessive to show what is its genotype iv. Cross heterozygote with homozygous recessive. Gametes t t T T tt tt 1. 1/2 of offspring are homozygous recessive v. Cross homozygous dominant with homozygous recessive Gametes t T T T 1. Get all heterozygotes 2. What you see is different if the unknown parent is homozygote or heterozygote a. Homozygote - you get on dwarf plants b. Heterozygote - you get 1/2 dwarf plants 2. Intermediate Inheritance or Incomplete dominance 4

5 a. What if neither trait is dominant? i. The heterozygotes are recognizable ii. Chickens 1. Black BB 2. White splashed B'B' 3. Andalusian Blue BB' 4. Heterozygote is "blending" of parental types Gametes B B' B BB black BB' blue B' BB' blue B'B' white b. Sex in Eutherian Mammals - doesn't go in Marsupials (Kangaroos etc.) i. Males have an odd little chromosome called the Y chromosome 1. Doesn t have much genetic information 2. Male genotype is XY ii. Females have two X chromosomes 1. Females are XX iii. Punnett square for sex chromosomes Gametes X Y X XX female XY male X XX female XY male 1. This system ensures that 50% of offspring are male and 50% female b. Mendel s second law - Law of Independent Assortment 1. Genes on different chromosomes assort independently during meiosis 2. Follow 2 genes with Punnett squares 3. Picture from book c. Multiple alleles 1... There can be many alleles for a particular gene 5

6 a. Each individual can have only 2 b. Chinchillas a. C - normal b. c ch chinchilla c. c h - Himalayan d. c - albino e. Phenotypes that are possible i. C/c h - Normal ii. c ch /c h - = chinchilla iii. c h /c = Himalayan iv. c/c = albino f. Note how complex it can get if there are lots of alleles in the population - Human eye color is an example c. Gene interactions a. Pleiotropy - Genes may have more than 1 effect on phenotype b. Polygenic traits i. Lots of genes determine phenotype 1. Height and skin color in humans 2. Result is what looks like smooth variation d. Sex-linked inheritance a. Color-blindness 6

7 i. Gene for colorblindness is on X chromosome 1. Alleles are normal vision (N) or color blind (n) 2. Female can be "carrier" of trait a. That is she can be heterozygous for it b. Let's do the Punnett squares with carrier and normal male: c. f Gametes N 0 e N m Nn normal N0 normal a female male N l Nn carrier N0 color- e female Blind male e. Linkage a. Genes are "linked" on same chromosome i. Ratios in offspring do not follow Mendelian ratios f. Crossing over a. Sometimes chromosomes exchange material during metaphase I in meiosis g. Chromosomal aberrations 7

8 a. Trisomy 21 Downs Syndrome i. Result from failure of chromosomes to separate during anaphase I in meiosis - called nondisjunction ii. Karyotyping c. Mutations i. Where do new alleles come from? ii. Genes are chemical structures on the chromosome iii. The chemical structure can be modified - 3 things can do this 1. Ionizing radiation 2. Certain chemicals 3. UV radiation - light iv. Also can be modified spontaneously - errors in copying v. These changes are inherited - mostly they are deleterious (bad) 1. Occasionally they are good or neutral - form new alleles vi. Mutations Get copied at reproduction 8