Bell Work Independent variable? Dependent variable?

Mendel and Meiosis Chapter 10

What do you see? People have noticed for thousands of years that family resemblances are inherited from generation to generation

Mendel’s Laws of Heredity How Genetics Began – Gregor Mendel considered the founder of Genetics Heredity – the passing on of characteristics from parents to offspring Traits – characteristics that are inherited Genetics - the branch of biology that studies heredity Link First studies of heredity in mid 19th century. Video is about 3 minutes long.

Mendel’s Laws of Heredity Sexual Reproduction – male and female sex cells called gametes fuse (fertilization) to produce a fertilized cell called a zygote which then develops into a seed. The transfer of pollen from a male reproductive structure to a female structure in a plant is called pollination

What is happening here? Making a “cross” Peas have both organs in the same plant enclosed in petals so they normally reproduce by self pollination. Making a “cross” Cross pollination: transferring pollen from one plant to another with different traits.

The Inheritance of Traits Mendel used true-breeding pea plants to study 7 traits Original plants = Parent (P) Offspring = F1 generation Offspring of parents with different forms of a trait = hybrid Offspring of F1 = F2 generation What is the F2 ratio? How many traits being studied? Crossing yellow seeded and Green seeded plants: True – breeding plants – populations of plants all with the same characteristic that for generations only produce plants with that characteristic. Monohybrid Cross – the two parents of a cross differed by a single trait. What would a dihybrid cross be?

Bell Work Identify the independent variables and dependent variables of an experiment based on this diagram.

Flower lab

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How did he explain his results? Different forms of a trait are called alleles Each trait controlled by two alleles Dominant form – appears in F1. Represented as a capital letter. Recessive form – hidden in F1. Represented as a lower case letter. In F2 generation, the ratio is 3:1 dominant to recessive

How does dominance work? Homozygous – both alleles for a trait are the same. Heterozygous – alleles for the trait are different. Only dominant trait can be seen. Y – yellow y- green YY Yy yY yy

Dominant or Recessive? ? ?

Genotype versus Phenotype Genotype – an organism’s actual allele pair. Phenotype – expression of an allele pair, or the way it looks or behaves. Y – yellow y- green YY Yy yY yy

Dominant or Recessive Homozygous or Heterozygous AA, Bb, cC, dd, EE, Ff, gg, HH, Ii, KK, mm A, b, c, D, E, f, G, h, J, K, m, n

B – brown, b blue eyes BB Genotype Phenotype

B – brown, b blue eyes Bb Genotype Phenotype

B – brown, b blue eyes bb Genotype Phenotype

B – brown, b blue eyes 4 Offspring: BB, Bb, Bb, bb Genotype Phenotype

B – brown, b blue eyes 4 Offspring: Bb, Bb, bb, bb Genotype Phenotype

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Law of Segregation The law of segregation states that different alleles for the same trait separate when gametes are formed. Thus, a mother that is heterozygous for brown eyes (Bb) could pass either a dominant brown allele (B) or a recessive blue allele (b) for eye color to her offspring. T t Sperm (gamete) Every individual has two alleles of each gene (trait) and when gametes are produced, each gamete receives one of the two alleles. Gametes are sex cells like egg and sperm During fertilization, the gametes randomly pair to produce four Egg (gamete)

Law of Independent Assortment The law of independent assortment states that when pairs of alleles separate, they do so independently of each other. Thus, the alleles for hair color and the alleles for eye color in humans are not inherited together. For example seed color AND seed shape.

Punnett Squares Developed to predict possible offspring of a cross between two known genotypes. Predict genotype and phenotype of offspring. Genotypic ratio? Phenotypic ratio?

Monohybrid Cross When parents with different forms of a gene (trait) are crossed, the offspring (F1) are heterozygous organisms called hybrids. A cross involving hybrids for a single trait is called a monohybrid cross. Their offspring (F2) will have a phenotypic ratio of 3:1

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Bell Work How many chromosomes does each body cell of a fruit fly have? How many do you think a sex cell would have?

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Chapter 10 Section 2: Meiosis

Meiosis Organisms have tens of thousands of genes that determine individual traits. How are they organized? Where?

Diploid and Haploid In body cells (somatic), chromosomes occur in pairs – one from each parent. Diploid: a cell with two of each kind of chromosome All body cells (somatic cells) Called 2n Gametes (sex cells) contain one of each kind of chromosome Called haploid 1n

Fertilization n=23 egg 2n=46 zygote The fusion of gametes (sperm and egg) forms a zygote (fertilized egg). This is called fertilization. n=23 egg sperm n=23 2n=46 zygote

Chromosomes – Bell Work If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA). Most organisms are diploid. Humans have 23 sets (pairs) of chromosomes. What is the diploid number of chromosomes for humans? What is the haploid number of chromosomes for humans?

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Haploid-Diploid Practice

Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size and normally contain identical arrangements of genes. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes Eye color locus Homologue Homologue 23

Why Meiosis? 2n=46 egg 4n=? zygote sperm 2n=46 What if mitosis was the only means of cell division? Division of a cell results in 2 identical daughter cells. If cell is 2n, what are daughter cells? If two 2n cells join, what is the resulting zygote? 2n=46 egg sperm 2n=46 4n=? zygote

Why Meiosis? n=23 egg 2n=46 zygote sperm n=23 Need a way to produce offspring with the same number of chromosomes as the parents. Requires cell division that produces ½ the number of chromosomes as a parent’s body (somatic) cell. n=23 egg sperm n=23 2n=46 zygote

Sexual Reproduction -production and fusion of haploid cells Sperm + Ovum (egg) Zygote fertilization The Zygote then develops by mitosis into a multicellular organism

1 Diploid (2n) cell  4 Haploid (n) cells Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis a diploid cell is reduced to 4 haploid cells called sex cells (eggs (ova) and sperm). These are the gametes. 1 Diploid (2n) cell  4 Haploid (n) cells Meiosis is two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes.

Cell Cycle and Mitosis Review

Tetrads In meiosis, homologous chromosomes line up, gene by gene forming a four-part structure called a tetrad. Each tetrad consists of two homologous chromosomes, each made up of two sister chromatids. eye color locus hair color Paternal Maternal

Bell Work H is dominant for hairy and h is recessive for nonhairy knuckles. HH Is this homozygous dominant, homozygous recessive or heterozygous? A cross between two heterozygous individuals will produce what genotype? What phenotype?

2n = 4 2n = 4 Sister chromatids tetrad Sister chromatids 2n = 4 n=2

Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I We need another division, each chromosome is still doubled!

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Prophase I Longest and most complex phase. 90% of the meiotic process is spent in Prophase I Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

During Prophase I “Crossing Over” occurs. During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. This creates variation (diversity) in the offspring’s traits. nonsister chromatids chiasmata: site of crossing over Tetrad variation

Metaphase I Can line up differently, results in diversity OR metaphase plate OR metaphase plate

Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Question: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

10 chromosomes (haploid) Answer: 10 chromosomes (haploid)

Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis metaphase plate

Anaphase II same as anaphase in mitosis sister chromatids separate

Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg

Telophase II

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Non-disjunction Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. Non-disjunction results with the production of zygotes with abnormal chromosome numbers

Non-disjunctions usually occur in one of two fashions. The first is called Monosomy, with only one chromosome. The second is called Trisomy, with 3 chromosomes.

Non-disjunction

Common Non-disjunction Disorders Down’s Syndrome – Trisomy 21 Turner’s Syndrome – Monosomy 23 (X) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18

Amniocentesis An Amniocentesis is a procedure a pregnant woman can have in order to detect some genetics disorders…..such as non-disjunction.

Karyotype (picture of an individual’s chromosomes) One of the ways to analyze the amniocentesis is to make a Karyotype What genetic disorder does this karyotype show? Trisomy 21….Down’s Syndrome

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Gene linkage and Maps Linked genes: those located close together on the same chromosome that are usually inherited together. How could genes on the same chromosome become separated? Crossing over produces new gene combinations and can be used to make chromosome maps Thomas Hunt Morgan