Chapter 10 Sexual Reproduction and Genetics

Two of these people are genetically related, can you tell which two?

Section 10.1 - Meiosis Main Idea : Meiosis produces haploid gamates. Meiosis is a type of cell division that reduces the number of chromosomes. 46 pairs reduced to 23 pairs A cell with half the number of chromosomes is called haploid. (23 chromosomes n) This type of cell division is important in the life cycle of organisms such as animals.

Why is meiosis cell division important? Human body cells have 46 chromosomes. Each parent gives half of their chromosomes (23) to their offspring. The chromosomes that make up a pair, one chromosome from each parent is called a homologous chromosome.

Passing on genetic traits Each chromosome consists of hundreds of genes. Genes are DNA segments that carry instructions for inherited traits such as eye color, hair color, and height

Examples of Inherited Traits Attached or unattached Can you roll your earlobes tongue? Some people can’t.

Hitchhiker thumb Widow’s peak

How genes are passed from generation to generation Organisms produce gamete cells (sex cells) that have half the number of chromosomes as body cells. (haploid – n chromosomes) The process of one parent’s gamete combining with another parent’s gamete is called fertilization.

Adults have 2n chromosomes (diploid cells – 46 each) If diploid cells fertilized each other, a cell would end up with 92 chromosomes. The next generation would have 184 chromosomes. Eventually there would be too many chromosomes in each cell

Instead… Animal cells undergo meiosis cell division to reduce the number of chromosomes before reproduction occurs. The offspring receives 23 chromosomes from the maternal (mother) gamete and 23 chromosomes from the paternal (father) gamete.

Meiosis involves two consecutive cell divisions called meiosis I and meiosis II Meiosis I – Reduction Stage Just like in mitosis, meiosis cell division begins with Interphase – cells undergo normal functions - DNA replication.

Meiosis I – Prophase I Chromatin coils up, Spindles appear, Nucleoli disappear, Synapsis occurs – pairs of homologous chromosomes form How is this a little different from mitosis prophase?

Meiosis I Prophase I Crossing over produces exchange of genetic information. Crossing over —chromosomal segments are exchanged between a pair of homologous chromosomes

Meiosis I Metaphase I Chromosome centromeres attach to spindle fibers. Homologous chromosomes line up at the equator. How is this different from mitosis metaphase?

Meiosis I Anaphase I Homologous chromosomes separate and move to opposite poles of the cell. Centromere not broken Chromosome number reduced from 2n to n How is this different from mitosis anaphase?

Meiosis I Telophase I The spindles break down. Chromosomes uncoil and form two nuclei The cell divides, (cytokinesis). Two daughter cells are produced with ½ the number of chromosomes. (n)

Meiosis II continues the cell division To begin the second set of meiosis stages, we start with the daughter cells from meiosis I. The DNA will not be replicated in meiosis II. If we separate the DNA and divide the cells, how many cells will be produced? How many chromosomes in each new cell?

Meiosis II – Prophase II A second set of phases begins as the spindle apparatus forms and the chromosomes condense.

Meiosis II – Metaphase II A haploid number of chromosomes line up at the equator.

Meiosis II – Anaphase II The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell.

Meiosis II – Telophase II The chromosomes reach the poles, and the nuclear membrane and nuclei reform.

Meiosis II - Cytokinesis Cytokinesis results in four haploid cells, each with n number of chromosomes.

Meiosis I and Meiosis II

Mitosis Meiosis I and II

Males – four sperm made by one cell dividing equally into four. Mitosis Meiosis I Meiosis II

Females – one ovum produced - Unequal division made during meiosis I and meiosis II Only the large cell survives and is passed on Extra cytoplasm provides food for the embryo.

The Importance of Meiosis I and II Meiosis consists of two sets of divisions Produces four haploid daughter cells that are not identical Results in genetic variation

Meiosis Provides Variation Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.

Studying a person’s genetic make up Every person is made up of trillions of cells, and each cell carries a full copy of that person’s genetic code, in the form of homologous chromosomes.

How are these karyotypes different?

Genetic defects – look closely, how are these karyotypes different?

Sexual Reproduction vs. Asexual Reproduction the organism inherits all of its chromosomes from a single parent. The new individual is genetically identical to its parent. Example – binary fission -- involves an equal division of both the organism cytoplasm and nucleus to form two identical organisms

Asexual Reproduction Vegetative Propagation Spores

Asexual Reproduction - Regeneration Body parts that are missing due to damaged or predation can be regrown.

Asexual reproduction - Budding The parent plant grows a structure (a bud) that matures, falls off and grows on its own.

Sexual reproduction Beneficial genes multiply faster over time. 2 parents - combination of genetic material not identical a) egg and sperm (gametes) combine

fertilization forms zygote zygote grows by mitosis -creates stem cells unspecialized cell mass of 100 –150 of the first cells to form.

Review of Section 10.1 Each student in class has characteristics passed on to them by their parents. The instructions for the traits are found on chromosomes in the nucleus. The DNA on the chromosomes is arranged in segments that control the production of protein These DNA segments are called genes. Each chromosome consists of hundreds of genes Each gene has an important role in determining the characteristics and function of the cell

10.2 Mendelian Genetics How Genetics Began The passing of traits to the next generation is called inheritance, or heredity. Mendel performed cross-pollination in pea plants. Mendel followed seven traits in the pea plants he bred. Examples: flower color, seed color, height of plant, texture of seed.

Self pollenating plant Cross pollinating plant

Mendel’s Experiments – cross-pollination by transferring a male gamete from the flower of one pea plant to the female reproductive organ in a flower of another pea plant.

The Inheritance of Traits The parent generation is also known as the P generation. Cross two pure parents one yellow & one green Green trait didn’t disappear, it was hidden or masked.

Mendel noticed that some pea plants produce yellow seeds and others produce green. The offspring of this P cross are called the first filial (F1) generation. = __?__ All yellow F1 self-pollinates = F2 ¾ yellow and ¼ green Green trait didn’t disappear, it was hidden or masked.

Genes in Pairs Mendel concluded there must be two forms of the seed trait in the pea plants Allele - An alternative form of a single gene passed from generation to generation The F1 pea plant had one yellow allele and one green allele Dominant – The allele for yellow seed color is dominant over the green seed color (Y) Recessive – the allele for green seed color is recessive (y)

Law of Dominance An organism with two of the same alleles for a particular trait is homozygous Homozygous yellow –seed plants are YY Homozygous green-seed plants are yy An organism with two different alleles for a particular trait is heterozygous.= Yy When alleles are present in the heterozygous state, the dominant trait will be observed.

Genotype and Phenotype You cannot tell an organisms alleles by its outward appearance. Some traits may be masked. An organism’s allele pairs are called its genotype A yellow plant may have a genotype of YY or Yy, both allele pairs will produce yellow seeds.

Mendel’s Law of Segregation Two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite. Heterozygous organisms are called hybrids

Law of Independent Assortment Random distribution of alleles occurs during gamete formation Genes on separate chromosomes sort independently during meiosis. Each allele combination is equally likely to occur.

Genotype and Phenotype The observable characteristic or outward expression of an allele pair is called the phenotype The phenotype of pea plants with the genotype of yy will be green seeds.

Genotype compared to Phenotype

Predicting Genotypes Punnett Squares and Probability Predict the possible offspring of a cross between two known genotypes Each box = 25% chance

Monohybrid Cross A cross that involves hybrids for a single trait is called a monohybrid cross. Four boxes in the Punnett square.

Dihybrid Cross is for two traits The inheritance of two or more traits in the same plant is a dihybrid cross. Dihybrids are heterozygous for both traits.

Punnett Square—Dihybrid Cross Four types of alleles (2 traits) from the male gametes and four types of alleles (2 traits) from the female gametes can be produced. The resulting phenotypic ratio is 9:3:3:1.

Cross Test Unknown dominant trait crossed with a known homozygous recessive, If offspring show recessive phenotype, It is know that the parent was heterozygous.

10.3 Gene Linkage and Polyploidy Genetic Recombination is the new combination of genes produced by crossing over and independent assortment Sex Chromosomes = One pair Female = XX and male = Xy

The X chromosome carries most genes to be passed to offspring. The X chromosome is the largest, Y being the smallest. The X chromosome carries genes needed by both males and females The Y chromosome carries mostly genes for male characteristics.

Co-dominance

Gene Linkage The linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently.

Incomplete Dominance

Chromosome Mapping Chromosome mapping = genes in exact locations on all chromosomes within organism farther apart they are more likely to cross over, so a higher percentage of crossed genes would mean the genes are farther apart

Chromosome map and gene linkage

After the crossover, A and B are together with c, and a and b are together with C on one of the two chromatids of the recombined chromosomes

Polyploidy Polyploidy is the occurrence of one or more extra sets of all chromosomes in an organism A triploid organism, for instance, would be designated 3n, which means that it has three complete sets of chromosomes. Strawberries are 8n

In humans, polyploidy is lethal, an embryo will not survive.