Mendel and Meiosis

Mendel’s Laws of Heredity The history of genetics Genetics – the study of heredity (the passing of characteristics from parents to offspring) Trait – characteristic that can be passed from parent to offspring Gregor Mendel – Father of Genetics, Austrian monk, studied garden pea plants. Made observations and gathered data on how pea plants passed traits from generation to generation. Applied math to his observations.

Mendel’s Laws of Heredity Mendel’s Experiments Eight years Chose the pea plant for three reasons: Reproduction methods – they reproduce sexually using gametes (sex cells). Structure – could allow him to isolate fertilization (union of gametes) to form a zygote (the fertilized cell). Self fertilization – pollen from the anther fertilizes the pistil from the same flower. Produces purebred plants. Purebred – when an organism receives the same genetic traits from both its parents.

Mendel’s Laws of Heredity Mendel also altered plants and transferred pollen by hand controlling pollination and preventing self fertilization. He crossbred plants producing hybrids. Hybrid – organism that receives different forms of a genetic trait from each parent. Distinctive traits – seven traits each with only 2 distinct forms. Ex. Pea pods either green or yellow. No blending or intermediate. Ex. Either tall or short, no medium. Rapid reproduction cycle – 90 days. Mendel got results quickly allowing him to repeat experiments many times to test his results.

Mendel’s Conclusions 1. Each trait controlled by a distinct “factor” (gene) each with two forms. 2. For every trait the pea plant must carry a pair of factors. 3. When a trait is inherited, the offspring receives one factor from each parent.

Mendel’s Conclusions Genes – sections of a chromosome that code for a trait. (2 copies, one from each parent) Allele – distinct form of a gene ( the different forms of Mendel’s factors) Dominant allele – form of a gene that is expressed when two different alleles are present. Recessive allele – form of a gene that is not expressed when paired with a dominant allele.

Principles of Inheritance The cellular basis of Inheritance Walter S. Sutton – outlined the Chromosome Theory of Heredity 1. The material of inheritance is carried by genes on chromosomes. 2. Specific genes are located on specific chromosomes (Thomas Boveri)

Principles of Inheritance Representing Alleles Use letters to represent alleles Uppercase letter for the Dominant allele (Ex. Y for yellow peas) Lowercase letter for the recessive allele *Ex. y – green peas) The uppercase and lowercase forms of the same letter are used because the alleles for yellow and green peas are tow versions of the same gene.

Genes Affect Traits Genotype – the genetic makeup of an organism, includes both genes ina homologous pair. Ex. Purebred yellow – YY, hybrid – Yy Phenotype – the outward expression of the trait. Ex. Yellow or green genotypes YY and Yy are both yellow phenotypes, yy genotype is a green phenotype.

Genes Affect Traits Homozygous – an organism in which the two alleles in a gene pair are identical Ex. YY – homozygous yellow; yy – homozygous green Heterozygous – an organism in which the two alleles for a particular trait are different. Ex Yy

Mendel’s Laws of Heredity Mendel’s Monohybrid Crosses Crossed the purebreds (parental generation – P) by hand (Ex. Green pods X yellow pods) Offspring called the first filial generation (F1) F1’s all yellow Allowed F1 plants to self – fertilize producing the F2 generation F2’s - 75% yellow and 25% green. 3:1 ratio Mendel repeated his experiments for all other traits and got the same results. F1’s were only one form of the trait – Mendel called this the co dominant form. He called the trait that did not appear in the F1 generation recessive. F2 generations showed that 75% had the dominant form and 25% had the recessive form of the trait. NO blending occurred.

Test Cross Scientists use test crosses to distinguish between homozygous dominant and heterozygous. Test cross – breed the organism whose genotype is unknown with a homozygous recessive organism. If the unknown genotype is heterozygous, about half the offspring should show the recessive phenotype. If the unknown genotype is homozygous dominant, all the offspring will show the dominant phenotype.

Predictions for Two Traits – Dihybrid Cross Mendel’s Law of Independent Assortment states that alleles will sort into all possible combinations. Ex. RrYy x RrYy (Parents heterozygous for round, yellow peas) Possible allele combinations in the gametes are RY, Ry, rY, ry List the possible gametes from one parent across the top of a 4x4 Punnett square, and the possible gametes from the other parent along the side of the Punnett square. Combine the alleles in the boxes to determine the possible offspring. Use FOIL

Predictions for Two Traits – Dihybrid Cross Mendel’s Law of Independent Assortment states that alleles will sort into all possible combinations. Ex. RrYy x RrYy (Parents heterozygous for round, yellow peas) Possible allele combinations in the gametes are RY, Ry, rY, ry List the possible gametes from one parent across the top of a 4x4 Punnett square, and the possible gametes from the other parent along the side of the Punnett square. Combine the alleles in the boxes to determine the possible offspring. Use FOIL

Mendel’s Laws 1. The Law of Segregation – each pair of genes segregates or separates during meiosis. Result – half of an organism’s gametes contain one gene from a homologous chromosome, and half of the gametes contain the other gene.

Mendel’s Laws 2. The Law of Independent Assortment – states that gene pairs segregate into gametes randomly and independently of each other. Mendel studied inheritance of two traits at the same time YYRR x yyrr. F1’s were allowed to self fertilize, the F2’s were all combinations of color and shape. The association of traits in the parents did not seem to matter. The behavior of the chromosomes in meiosis – the separation of chromosome pairs occurs randomly and produces all possible combinations of chromosomes in the gametes. If the chromosome pairs did not separate randomly, offspring would have the same combination of traits as one of the parents.

Mendel’s Laws 3. The Law of Dominance – the dominant allele is expressed and the recessive allele can be hidden.

Meiosis Sexual Reproduction Involves the fusion (union) of 2 specialized reproductive cells called gametes. Fertilization (fusion of gametes) produces a zygote. Female gamete – egg. Male gamete - sperm

Meiosis Chromosome Number Every body cell in an organism has the same number of chromosomes. Every normal member of a species has the same number of chromosomes Chromosomes occur in pairs that are identical in size, shape, and gene arrangement. Matching pairs are called homologous chromosomes. One from each parent Human body cells have 46 chromosomes or 23 homologous pairs Diploid (2n) – having both members of each homologous pair. Two complete sets of chromosomes. Human diploid number is 46 Haploid (n) – having one member of each homologous pair. One set of chromosomes. Human haploid number is 23.

Meiosis Meiosis Also called reduction division Reduces the number of chromosomes in gametes by half Purpose – to form gametes with the haploid number. Separates homologous chromosomes. Two division: Meiosis I – separation of homologous chromosomes. Meiosis II – separation of sister chromatids.

Meiosis I Interphase I – replication of DNA Prophase I – pairing of homologous chromosomes to form tetrads. Each tetrad consists of 4 chromatids. Synapsis, the twisting together of tetrad chromatids, leads to crossing over. Crossing over between homologous chromosomes produces genetic recombination (reassortment of chromosomes).

Meiosis I Metaphase I – tetrads line up along the equator. Anaphase I – homologous chromosomes separate. Sister chromatids remain connected at the centromere. Telophase I – cell divides producing two haploid daughter cells.

Changes in the genome. Nondisjunction – the failure of chromosomes to separate during cell division. When it occurs in mitosis, the cell may die but the organism is not harmed. However, in meiosis nondisjunction can create abnormal gametes which can produce abnormal offspring.

Changes in the genome Monosomy – having one copy of a particular chromosome instead of two. Trisomy – having three copies of a particular chromosome instead of two. In humans having monosomy or trisomy is so disruptive that the embryo dies; but when it occurs in certain chromosomes, the embryo survives but has developmental difficulties.

Changes in the genome Down syndrome – trisomy 21 Turner syndrome – monosomy X (XO) Klinefelter Syndrome - XXY

Changes in the genome Polyploidy – nondisjunction occurs in all chromosome pairs. In animals, polyploidy almost always results in death. In plants, it can produce more robust plants. Many of the plants we eat are from polyploid plants. Ex. Potatoes, peanuts, oats, bananas, plums, apples, coffee, sugarcane, cotton.

Meiosis II – sister chromatids separate Prophase II – brief, almost nonexistent. No replication occurred. Metaphase II – chromatids line up along the equator. Spindle fibers attach to the centromere. Anaphase II – centromeres divide, sister chromatids separate. Chromosomes move to opposite poles. Telophase II – meiosis completed. Result – 4 haploid cells (gametes)

Predictions for One Trait – Monohybrid Cross Probability can be represented as a fraction (1/2) or a ratio (1:1) Punnett Square – a grid for organizing genetic information; it shows probabilities, not actual results/ Punnett squares can be used to predict the offspring of crosses between two organisms. Top and sides – gamete combinations Inside squares – possible offspring