Transcript Chapter 10 Mendel and Meiosis

Chapter 10 Mendel and
Meiosis
Objectives: Analyze the the results obtained
by Gregor Mendel in his experiments with
garden peas.
Predict the possible offspring of a genetic
cross by using a Punnent square.
Standard 4
. apply the principles of Mendelian
inheritance to make predictions about
offspring.
• identify dominant and recessive traits,
given the results of a monohybrid
cross in a scenario.
• distinguish between dominant and
recessive traits, given the results of a
monohybrid cross.
• determine the genotype and
phenotype of a monohybrid cross,
given a Punnett square.
• diagram and analyze a
monohybrid cross, given a genetic
problem.
• Father of Genetics—
•
Gregor Mendel
Section 1
• Mendel carried out the first important
studies of pea plants,
• the passing on of characteristics from
parents to offspring.
• Genetics is the branch of biology that
studies heredity.
• Characteristics that are inherited are
called traits.
• Mendel used the pea plants to study
genetics because they reproduce
sexually with two distinct cells.
• Sex cells are called gametes.
• Ways Mendel carefully controlled his
experiments and the peas he used
were: He studied only one trait at a
time to control variables and his plants
were true-breeding plants.
• A hybrid is the offspring of parents that
have different forms of a trait, such as
tall and short.
• His first experiments are called
monohybrid crosses.
• First generation
• TT X tt
• Second generation
• Tt x Tt
• In every case, Mendel found that one
trait of a pair disappeared in the F1
generation only to reappear in the next
generation.
• Some traits Mendel chose for his
experiments were: seed shape, seed
color, flower color pod color , plant
height.
• Each organism has two factors that
control each of his traits.
• We call these different gene forms alleles.
• .An organism’s two alleles are located on
different copies of a chromosome—inherited
from the female parent and the male parent.
• Mendel called the observed trait the
dominant trait and the disappeared trait
recessive.
• An upper case letter is used for the dominant
trait and a lower case letter is used for the
recessive trait.
• The dominant allele is written first.
• Two alleles for each trait must separate
when a zygote is formed. A parent,
therefore, passes on at random only
one allele for each trait to each
offspring.
• The way an organism looks and
behaves is called its phenotype.
• The phenotype of a tall plant is tall
whether it is TT or Tt.
• The gene combination an organism
contains is known as its genotype .The
genotype of a tall plant that has two
alleles for tallness is TT or Tt.
• An organism is homozygous for a trait
if its two alleles for the trait are the
same. Which would be written TT or tt
• For tall or short plants.
• An organism is heterozygous for a trait
if its two alleles for the trait differ from
each other. Which would be written Tt.
• Punnett SquaresA short hand way of finding the
expected proportions of possible
genotypes in the offspring of a cross.
• This Punnett Square
shows how we can
diagram the genes.
• The orange bird has
two dominant
Agenes.
• We put two A s
along the top of the
square.
•
• The blue bird has
two recessive a
genes.
• We put two a s
down along the left
side of the square.
• All the offspring have the genes
Aa.
They will all have orange feathers,
but will carry a recessive gene for
blue feathers.
This is called the F1 generation.
Now suppose that two individuals
from the F1 generation become
parents
.
•
•
•
•
•
•
Here they are!
The baby birds are called the F2
generation.
You can see how their genes work
out.
The offspring are coded in the
squares.
One bird will be orange with two AA
genes.
•
Two birds will be orange with genes
coded Aa.
One bird will be blue and will have
two recessive aa genes.
•
Individual nests of birds may not
turn out exactly like this, but if there
are many baby birds, they will work
out genetically with the
•
ratios 1:2:1.
Cross between Heterozygous (Bb) parents
In this case, the probablity of a child having bb is ?
• If you know the genotypes of the
parents, you can predict the
possible genotypes of their
offspring.
•
•
•
•
•
Monohybrid crosses.
Give phenotype and genotype.
Cross Tt x Tt
Cross TT x tt
Cross Tt x tt
What kind of cross is this?
Dihybrid Cross
• A cross involving two different traits is
called a dihybrid cross.
• Mendel took true-breeding pea plants
with round and yellow seeds and
crossed them with true breeding green
and wrinkled seeds.
• Look at this cross and see results.
• In the F1 generation he found all seeds
to be heteorzygous round and yellow
seeds.
• He let the F1 generation plants to
pollinate themselves.
• Show this cross.
• Mendel’s second law states that
genes for different traits are
inherited independently of each
other. It is known as law of
independent assortment.
Section 2
Meiosis
• Objectives:
• Analyze how meiosis maintains a
constant number of chromosomes
within a species.
• Infer how meiosis leads to variation in a
species.
• Relate Mendel’s laws of heredity to the
events of meiosis
• Organisms have tens of thousands of
genes that determine individual traits.
• In the body cells of animals and most
plants , chromosomes occur in pairs.
• A cell with two of each kind of
chromosomes is called a diploid cell
and is said to contain a diploid, or 2n ,
number chromosomes.
• Organisms produce gametes that
contain one of each kind of
chromosome, a cell with one each kind
of chromosome is called a haploid cell
and is said to contain a haploid or n.
• Mendel’s conclusion was that parent
organisms give one factor, or allele, for
each trait to each of their offspring.
Meiosis
• A human body cell contains 23 pairs of
chromosomes. The gametes - sperm or
eggs - contain half this number of
chromosomes, which is why meiosis is
sometimes called 'reduction division'
During prophase I
• homologous chromosomes pair and form
synapses, a step unique to meiosis.
Metaphase I
•
•
•
•
chromosomes
(four chromatids)
align at the
metaphase plate.
•
•
•
•
•
•
•
•
•
•
•
Anaphase I
Chromosomes, each with
two chromatids, move to
separate poles. Each of the daughter
cells is now
haploid (23
chromosomes),
but each
chromosome
has two
chromatids.
Telophase I
•
Nuclear envelopes
may reform, or the
cell may quickly
start meiosis II.
• A human cell has 46 total or 23 pairs of
chromosomes. Following mitosis, the
daughter cells would each have a total of
______ chromosomes. After meiosis I, the
two daughter cells would have
_____chromosomes, and after meiosis II
______ chromosomes.
• A.46, 46, 46B.46, 23, 23 C.23, 23, 23D.46,
12, 12
• The two chromosomes of each
a pair in a diploid cell help
determine what the individual
organism looks like. The
paired chromosomes are called
homologous chromsomes.
Why meiosis?
• When cells divide by mitosis, the new cells
have exactly the same number and kind of
chromosomes as the original cells.
• Meiosis occurs in body cells of each parent
that produce gametes.
• Meiosis consists of two separate divisions.
By the end of meiosis II, there are four
haploid cells
• Male gametes -- sperm
• Female gametes -- egg
• Fertilized egg --zygote
• Fusion of haploid sex cells is called sexual
reproduction.
• Crossing over- exchange of genetic material.
• Cells that are formed by mitosis are identical
to each other and to the parent cell. Meiosis
however, provides genetic variation.
• The reassortment of chromosomes and the
genetic information they carry, either by
crossing over or by independent segregation
of homologous chromosomes, is called
genetic recombination.
• It is a major source of variation among
Organisms.
Mistakes in Meiosis
• The failure of homologous
chromosomes to separate properly
during meiosis is called
nondisjunction.
• Trisomy- extra chromosome
• Monosomy- lacks a chromosome
• Triploidy- Gamete with an extra set of
chromosome