Ch. 10 Sexual Reproduction and Genetics

p. 270 -

10.1 Meiosis

p. 270 - 276

Essential Question

Main Idea !  Meiosis produces haploid gametes

Where are the instructions for each trait located in a cell?

!  On the DNA inside the nucleus

Genes !  DNA segments !  Genes are segments of DNA on a

chromosome in the nucleus of a cell

How many chromosomes do human body cells have?

!  46

How many do each parent contribute?

!  23

Homologous Chromosomes !  Chromosomes that make up a pair, one

chromosome from each parent

Gametes !  Sex cells (egg and sperm) !  Contain ½ the number of chromosomes

How many chromosomes do human gametes have?

!  23 or n

Haploid !  A cell with ½ the number of

chromosomes of a diploid cell !  Sex cells are haploid !  Represented by n

Fertilization !  When a haploid gamete joins with

another haploid gamete

How many chromosomes does a fertilized cell have?

!  46 or 2n

Diploid !  Having 2 copies of each chromosome

Meiosis !  Formation of gametes !  Cell division that reduces the number of

chromosomes by separating homologous chromosomes

!   Involves 2 stages; Meiosis I and Meiosis II

Meiosis I - Interphase !  Chromosomes replicate !  Chromatin condenses

Meiosis I – Prophase I !  Pairing of homologous

chromosomes !  Each chromosome

consists of two chromatids

!  Nuclear envelope breaks down

!  Spindles form

Meiosis I – Prophase I !  Crossing over – segments of

chromosomes are exchanged between a pair of homologous chromosomes

!  The results of crossing over are new kinds of genes

Meiosis I – Metaphase I !  Chromosome centromeres attach to

spindle fibers !  Homologous chromosomes line up at

the equator

Meiosis I – Anaphase I !  Homologous chromosomes separate

and move to opposite poles of the cell

Meiosis I – Telophase I !  Spindles break down !  Chromosomes uncoil and form two

nuclei !  Cell divides

End of 10.1 I

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 !  Sister chromatids are pulled apart at the

centromere by spindle fibers and move toward the opposite poles of the cell

Meiosis II – Telophase II !  Chromosomes reach the poles and the

nuclear membrane and nuclei reform

Meiosis II - Cytokinesis !  Results in four haploid cells, each with n

number of chromosomes

Mitosis vs. Meioisis !   One division !   Two identical cells

formed !   Daughter cells

genetically identical !   Occurs only in body

cells !   Involved in growth

and repair

!   2 sets of divisions !   4 haploid cells

formed !   Daughter cells NOT

genetically identical !   Occurs in

reproductive cells !   Involved in

production of gametes

How does meiosis provide variation?

1.  Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result

2.  Crossing over

Where does an asexual organism get its chromosomes?

!  The organism inherits all of its chromosomes from a single parent

Why is sexual reproduction beneficial?

!  Beneficial genes multiply faster over time than they do when reproduction occurs asexually.

10.2 Mendelian Genetics

p. 277 - 282

Essential Question

Main Idea !  Mendel explained how a dominant allele

can mask the presence of a recessive allele

What is heredity? !  The passing of traits to the next

generation !  Also called inheritance

How did Mendel perform cross pollination in pea plants?

!  By transferring a male gamete from the flower of one pea plant to the female reproductive organ in a flower of another pea plant

Genetics !  The science of heredity

What is the P generation? !  The parent generation !  Yellow seeds (YY) crossed with green

seeds (yy)

What is the F1 generation? !  The offspring of the P cross !  All appeared yellow (Yy)

What is the F2 generation? !  The offspring from the F1 cross !  Most (3/4) appeared yellow (YY or Yy)

and some (1/4) appeared green (yy)

What did Mendel conclude? !  That there must be two forms of the

seed trait in pea plants – yellow and green

allele !  An alternative form of a single gene

passed from generation to generation !  Example: pea color (yellow or green),

pea shape (round or wrinkled)

Dominant !  A trait that appears !  Denoted by an uppercase letter

Recessive !  A trait that is masked !  Denoted by a lowercase letter

Example: !   If yellow (Y) is dominant and green

(y) is recessive, what color will the pea be?

1.  YY 2.  Yy 3.  yy

Homozygous !  An organism with two of he same

alleles for a particular trait !  YY !   yy

Heterozygous !  An organism with two different alleles

for a particular trait !  Yy

Genotype !  An organism’s allele pairs

!  YY, Yy, or yy

Phenotype !  The observable characteristic

!  Yellow or green

Law of Segregation !  The two alleles for each trait separate

during meiosis !  During fertilization, two alleles for that

trait unite

Hybrids !  A heterozygous organism !  Yy

Monohybrid cross !  A cross that involves hybrids for a single

trait !  Three possible genotypes: YY, Yy, yy !  Genotypic ratio is 1:2:1 !  Phenotypic ratio is 3:1

Dihybrid cross !  The simultaneous inheritance of two or

more traits in the same plant !  They are heterozygous for both traits

!  4 types of alleles: Y_R_, yyR_, Y_rr, yyrr !  Phenotypic ration (9:3:3:1)

Law of independent assortment

!  Random distribution of alleles occurs during gamete formation

!  Genes on separate chromosomes sort independently

Punnett Squares !  Used to predict the possible offspring of

a cross between two known genotypes

Main Idea !  The crossing over of linked genes is a

source of genetic variation

Genetic Recombination !  The new combination of genes

produced by crossing over and independent assortment

Polyploidy !   Is the occurrence of one or more extra

sets of all chromosomes in an organism !  1 in 3 plants are polyploid

!  Helps with vigor and size

End of Ch. 10