CHAPTER 10
MENDEL AND MEIOSIS
WHAT YOU’LL LEARN:
**THE BASIC CONCEPT OF GENETICS
**THE PROCESS OF MEIOSIS
WHY IT’S IMPORTANT:
**YOU INHERITED TRAITS FROM YOUR
PARENTS.
**IF YOU KNOW MEIOSIS YOU WILL SEE HOW
TRAITS ARE PASSED TO OFFSPRING.
10.1 MENDEL’S LAWS OF HEREDITY
▸I. Why Mendel Succeeded
–**Mendel lived in a monastery in the mid
1800's
–**Heredity - the passing of characteristics
from parents to offspring
–**Genetics - the study of heredity.
A. Mendel chose his subject carefully
▪1. Mendel chose garden peas.
▪2. Gametes - distinct male and female sex
cells
▪3. Pollination - transfer of pollen to pistil
▪4. Fertilization - uniting of male and female
▪
gametes
▪B. Mendel was a careful researcher
▪
▪
He studied one trait at a time to control
variables
II. Mendel’s Monohybrid Crosses
** Hybrid - the offspring of parents
that have different forms of a trait
**Monohybrid - One trait
▪ A. The first generation
▪P
+
=>
▪ B. The second generation
▪ F1
The Rule of United Factors
▸1. Each organism has two factors to control each trait.
▸2. These factors we call genes they are located on
the chromosomes.
▸3. Alleles - different forms of the same gene.
The Rules of Dominance
▸1. Dominant - Trait expressed without exception
▸2. Recessive - Trait that may be hidden by the
dominant trait.
▸3. Symbols: Capital for Dominant
Lower case for recessive.
▪ E. The
law of Segregation
▪ III. Phenotypes and Genotypes
▸1. Phenotype - The way an organism looks and behaves.
▸2. Genotype - The gene combination an organism
contains.
▸3. Homozygous - The two alleles for the trait are the
same.
( TT or tt )
▸4. Heterozygous - The two alleles for the trait are
different.
( Tt )
▪ IV. Mendel’s Dihybrid Crosses
▸
Organisms that differ from each other by two traits
▪ V. Punnett Squares
▸ A shorthand way of finding the expected proportions
of a cross.
▪ VI. Probability
▸The likelihood that a trait will be shown
▪ 10.2 Meiosis
▸A. Diploid and haploid cells.
– 1. Chromosomes are in pairs
- one from the mom and one from the dad.
– 2. Diploid - A cell with two of each kind of chromosome ( 2N )
– 3. Haploid - one of each kind of chromosome, these are gametes
(N)
▪ B. Homologous Chromosomes
▸ Pair of chromosomes that have genes for the same traits.
May not be identical alleles.
▪ C. Why Meiosis
▸ 1. Produces gametes containing half the number of
chromosomes as a parent body cell has.
▸ 2. Male gametes are called sperm.
▸ 3. Female gametes are called eggs.
▸ 4. A zygote results when an Egg and a Sperm unite.
This is sexual Reproduction
▪ III The phases of Meiosis
▸A. Interphase
– 1. Cell replicates its chromosomes
=>
– 2. Has two identical sister chromatids
▪ B. Prophase I
▸1. Each pair of homologous chromosomes comes together
▸2. Crossing over - chromosomes can exchange
genetic material
▸3. In humans the average is two to three crossovers
per pair.
▪ C. Metaphase I
▸The Homologous pairs of chromosomes line up along the
equator
▪ D. Anaphase I
▸ The homologous pairs of chromosomes separate.
▪ E. Telophase I
▸Cytoplasm divides to yield two new cells with half the
number of chromosomes
Meiosis Provides for Genetic Variations
▸ A. genetic recombination - The
reassortment of genetic information
resulting from crossing over and
independent segregation.
▸ B. Meiosis Explains Mendel’s results
▪ IV. Mistakes in Meiosis
▸Nondisjunction - failure of homologous chromosomes
to separate properly.
▸A. Trisomy - one extra chromosome (Downs Syndrome)
XXX
▸B. Monosomy - one less chromosome. In humans most
zygotes with Monosomy do not survive.
(Turners Syndrome) XO
▸C. Triploidy - Inherits an additional whole set of
chromosomes.
▸D. Polyploidy - more than the usual sets of chromosomes
– rare in animals, many in plants