Sexual Reproduction and Genetics

Chapter 10

Meiosis

Meiosis - produces haploid gametes, reduction division because it reduces the number of chromosomes

The DNA on chromosomes are arranged into segments called genes

Each chromosome has about 1500 genes that help determine characteristics and functions of each cell

Humans have 46 chromosomes, each parent contributes 23 chromosomes, so there are 23 pairs of chromosomes

Meiosis

Homologous chromosomes - each pair of chromosomes that contain a chromosome from each parent

These pairs have the same length, and have their centromere in the same position, and they carry the same genes as each other

Terms

Gametes - sex cells that have half the number of chromosomes, in humans each gamete has 23 chromosomes

Haploid - a cell with n number of chromosomes

Fertilization - one haploid gamete combines with another haploid gamete, this will create a total of 2n chromosomes (n chromosomes from the male and n chromosomes from the female)

Terms

Diploid - a cell with 2n number of chromosomes

Crossing over - chomosomal segments are exchanged between a pair of homologous chromosomes

Somatic Cells – diploid sex cells (aka gametes)

Meiosis I

Interphase - chromosomes replicate, and chromatin condenses, carries out normal cell processes

Prophase I - Pairing of Homologous chromosomes occurs each consists of 2 chromatids, genetic information is passed by crossing over, nuclear envelope is broken down, spindles form

Metaphase I - Chromosome centromeres attach to spindle fibers, Homologous chromosomes line up at the middle

Anaphase I - Homologous chromosomes separate and move to opposite poles of the cell

Telophase I - The spindles break down, Chromosomes uncoil and form two nuclei, the cell divides

Meiosis II

Prophase II - Chromosomes condense, spindles form in each new cell, spindle fibers attach to chromosomes

Metaphase II - Centromeres of chromosomes line up randomly at the equator of each cell

Anaphase II - Centromeres split, Sister chromatids separate and move to opposite poles

Telophase II - Four nuclei form around chromosomes, spindles break down, cells divide

Products - Four cells have formed, each nucleus contains a haploid number of chromosomes

Meiosis

Meiosis provides variation - the pairs of Homologous pairs that line up in the center of the cell is completely random, depending on how they line up determines which gametes get what set of characteristics, this is why we are all different

Probability

Probability - inheritance of genes is similar to flipping a coin

The probability of the coin landing heads in 1 out of 2 or 1/2. If the same coin is flipped twice, the probability of it landing on heads is 1/2 each time or 1/4 for both times.

Sexual Vs. Asexual Reproduction

During Asexual reproduction the organism inherits all of its chromosomes from a single parent, so they are genetically identical to its parent

Mendelian Genetics

Gregor Mendel - considered the father of genetics, he studied traits in pea plants

Genetics - the science of heredity

Mendel took the male gamete from a true breeding green pea plant, and cross-pollinated with the female gamete of a true breeding yellow pea plant. He cut off the male parts of the yellow plant to prevent self pollination. The yellow and green seed plants are called the Parent Generation or the P generation

The Experiment

All of the resulting offspring had yellow seeds, this is known as the F1 generation (First Filial)

The green seed trait seemed to disappear in the F1 generation

Mendel then planted the F1 seeds and allowed them to self pollinate, and green seeds then appeared, this is known as the F2 generation (Second Filial)

Of the seeds that were collected in this experiment there were 6022 yellow seeds, and 2001 seeds, almost exactly a 3:1 ratio

Genes in Pairs

Allele - an alternative form of a single gene passed from generation to generation ex. the yellow and the green colored seeds, each of these is a

different form of a single gene so the allele would be color ex. a Ford F150 and a Dodge 1500, these are different brands

but they are both trucks so the allele would be a truck

Dominant traits - the trait that appeared in the F1 generation ex. the yellow seeds

Recessive traits - the trait that was masked in the F1 generation ex. the green seeds

Dominance

Since the yellow allele was dominate it can be represented by a capital Y

Since the green allele was recessive it can be represented by a lower case y

Homozygous - when the alleles are the same ex. YY is yellow or yy is green

Heterozygous - when the alleles are different, the dominate trait will be observed but it will be a carrier of the recessive gene ex. Yy is yellow

Terms

Genotype - allele pairs ex. YY or Yy

Phenotype - the observable outward appearance ex. green seeds

Mendel's Law of Segregation - two alleles for each trait separate during meiosis, and unite during fertilization

Hybrids - when gametes are united, the F1 generation are all hybrids of the two gametes from the P generation

Genetic Crosses

Monohybrid Cross - a cross that involves hybrids for a single trait ex. Yy produces two types of gametes male and female and each either

has a Y or y allele▪ This is a random event, and can occur as YY, Yy, Yy, or yy

Dihybrid Cross - a cross that involves hybrids for two or more traits in the same plant

In pea plants R is for round seeds (dominate) and r is for wrinkled seeds (recessive)

Y is for yellow seeds (dominate) and y is for green seeds (recessive) ex. YYRR x yyrr - all of these would be YyRr yellow round seeds

Law of independent assortment - a random distribution of alleles occurs during gamete formation ex. YR Yr yR yr each is equally likely to occur

Punnett Squares

Monohybrid - rolling your tongue ex. if you can it is dominate T

suppose both of your parents can do it and they have the heterozygous geneotype

Example

Dihybrid Cross

Dihybrid Cross - YYRR x yyrr F1 - female YyRr x male YyRr

Example