Download Unit 07 - Lessons 4-6

6.6 Meiosis and Genetic Variation
KEY CONCEPT
Independent assortment and crossing over during
meiosis result in genetic diversity.
6.6 Meiosis and Genetic Variation
!   Sexual reproduction creates unique combinations of
genes.
•  Sexual reproduction creates unique combination of genes.
–  independent assortment of chromosomes in meiosis
–  random fertilization of gametes
•  Unique phenotypes may give a reproductive advantage to
some organisms.
6.6 Meiosis and Genetic Variation
!   Crossing over during meiosis increases genetic diversity.
•  Crossing over is the exchange of chromosome
segments between homologous chromosomes.
–  occurs during prophase I of meiosis I
–  results in new combinations of genes
6.6 Meiosis and Genetic Variation
•  Chromosomes contain many genes.
–  The farther apart two genes are located on a
chromosome, the more likely they are to be separated
by crossing over.
–  Genes located close together on a chromosome tend to
be inherited together, which is called genetic linkage.
•  Genetic linkage allows the distance between two genes to
be calculated.
S-B-8-1_Non-Mendelian Heredity PowerPoint
6.6 Meiosis and Genetic Variation
!   Linked Traits
•  When the loci of two
genes are located close
together on the same
chromosome
–  Red hair, freckles,
and fair skin are
linked.
http://en.wikipedia.org/wiki/File:Woman_redhead_natural_portrait_1.jpg
7.1 Chromosomes and Phenotype
KEY CONCEPT
The chromosomes on which genes are located can
affect the expression of traits.
7.1 Chromosomes and Phenotype
!   Two copies of each autosomal gene affect phenotype.
•  Mendel studied autosomal
gene traits, like hair texture.
7.1 Chromosomes and Phenotype
•  Mendel s rules of inheritance apply to autosomal genetic
disorders.
–  A heterozygote for a recessive disorder is a carrier.
–  Disorders caused by dominant alleles are uncommon.
(dominant)
7.1 Chromosomes and Phenotype
!   Males and females can differ in sex-linked traits.
•  Genes on sex chromosomes are called sex-linked genes.
–  Y chromosome genes in mammals are responsible for
male characteristics.
–  X chromosome genes in mammals affect many traits.
7.1 Chromosomes and Phenotype
•  Male mammals have an XY genotype.
–  All of a male s sexlinked genes are
expressed.
–  Males have no
second copies of
sex-linked genes.
7.1 Chromosomes and Phenotype
•  Female mammals have an XX genotype.
–  Expression of sex-linked genes is similar to autosomal
genes in females.
–  X chromosome inactivation randomly turns off one X
chromosome.
S-B-8-1_Non-Mendelian Heredity PowerPoint
7.1 Chromosomes and Phenotype
!   Sex-Linked Traits
•  Genes located on a
sex (X or Y)
chromosome
♀XX & ♂XY
–  Some forms of
hemophilia
–  Red-green
colorblindness
http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/sexlinked%20notes.htm
S-B-8-1_Non-Mendelian Heredity PowerPoint
Barr Bodies
•  The inactive X
chromosome found
in female (XX)
somatic cells
http://www.genetics.com.au/factsheet/fs14.asp
The nucleus of a cell. The arrow is
pointing to the inactive X
chromosome, the Barr Body.
–  Not found in normal
males (XY) .
–  Explains calico
coloring in cats.
S-B-8-1_Non-Mendelian Heredity PowerPoint
7.1 Chromosomes and Phenotype
!   Calico Cats
http://waynesword.palomar.edu/biex4hnt.htm S-B-8-1_Non-Mendelian Heredity PowerPoint
7.1 Chromosomes and Phenotype
!   Solving Sex-Linked Problems
A woman who is a carrier for red-green
colorblindness marries a man with normal sight.
What percent of the male and female children will
be red-green colorblind or carriers?
STEP 1:
♀ XX'
♂ XY
STEP 2:
STEP 3:
STEP 4:
X, X'
X, Y
STEP 5:
X
X
Y
XX
XY
X'
XX' X'Y
STEP 6:
50% ♀ Normal
50% ♀Carriers
50% ♂Normal
50% ♂ Colorblind
S-B-8-1_Non-Mendelian Heredity PowerPoint
7.1 Chromosomes and Phenotype
!   THINK-PAIR-SHARE
1
Wild Type
2
Homozygous
7
3
8
4
5
6
Heterozygous
1.  Is the trait sex-linked? How do you know?
2.  How could we tell if female #5 is heterozygous
if she never has children?
3.  What are the expected phenotypic ratios from a
cross between #3 & #7?
http://upload.wikimedia.org/wikipedia/commons/c/c5/Autosomal_Recessive_Pedigree_Chart_.svg
7.2 Complex Patterns of Inheritance
!   Phenotype can depend on interactions of alleles.
•  In incomplete dominance, neither allele is completely
dominant nor completely recessive.
–  Heterozygous phenotype is intermediate between
the two homozygous phenotypes
–  Homozygous parental phenotypes not seen in F1
offspring
S-B-8-1_Non-Mendelian Heredity PowerPoint
Incomplete Dominance
•  Heterozygotes look
different from a
homozygotes
•  Blending of
phenotypes
RR x RIRI
RRI
http://chsweb.lr.k12.nj.us/psidelsky/Chapter10(Gen).htm
7.2 Complex Patterns of Inheritance
•  Codominant alleles will both be completely expressed.
–  Codominant
alleles are
neither
dominant nor
recessive.
–  The ABO blood
types result
from
codominant
alleles.
•  Many genes have more than two alleles.
Codominance: Both alleles of a gene
pair are expressed.
• 
• 
• 
Example: Human Blood Types
Human blood type is determined by codominant
alleles. There are three different alleles: IA, IB, and i.
The IA and IB alleles are codominant, and the i allele
is recessive.
The possible human phenotypes for blood group are
type A, type B, type AB, and type O.
Type A and B individuals can be either homozygous
(IAIA or IBIB, respectively), or heterozygous (IAi or
IBi, respectively). Type O is IOIO.
S-B-8-1_Non-Mendelian Heredity
PowerPoint
S-B-8-1_Non-Mendelian Heredity PowerPoint
THINK-PAIR-SHARE
1. 
2. 
3. 
Distinguish between incomplete dominance and
codominance.
How many phenotypes are seen when codominance
is possible?
Red color (R) in snapdragons shows incomplete
dominance to white snapdragons (RI). The
heterozygotes are pink. If pink snap dragons are
crossed with a white snapdragon, what are the
genotypic/ phenotypic ratios?
R
RI
RI
RRI RIRI
S-B-8-1_Non-Mendelian Heredity PowerPoint
Problems with Multiple Alleles
A woman with type O blood marries a man with
type AB blood. What are the possible blood
types that a child of theirs could inherit?
STEP 1:
♀ ii
STEP 2:
STEP 3:
STEP 4:
i
STEP 5:
♂ IA I B
I A , IB
i
IA
IB
IAi
IBi
STEP 6: There s a 50% chance that their baby will be
Type A or Type B.
S-B-8-1_Non-Mendelian Heredity PowerPoint
7.2 Complex Patterns of Inheritance
!   THINK-PAIR-SHARE
Black fur (BBl) is completely dominant to brown fur in
rabbits, but brown fur (BBr) is dominant to white (b). A
hybrid brown doe (BBrb) is crossed with a hybrid black buck
(BBlb). What are the expected phenotypes of the offspring?
Ratios?
7.2 Complex Patterns of Inheritance
!   Many genes may interact to produce one trait.
•  Polygenic traits are
produced by two or
more genes.
Order of dominance:
brown > green > blue.
S-B-8-1_Non-Mendelian Heredity PowerPoint
Polygenic Inheritance
http://www.nature.com/nrg/journal/v10/n12/box/nrg2670_BX1.html
Poly-many genic-gene
•  More than one gene
on more than one loci
or on more than one
chromosome.
•  Give the appearance
of gradation
–  Hair & skin color
–  Height
–  Body size
http://scienceblogs.com/geneticfuture/a_ruler_beats_genetics_in_pred/tall_short1.jpg
7.2 Complex Patterns of Inheritance
!   The environment interacts with genotype.
•  Phenotype is a combination
of genotype and
environment.
•  The sex of sea turtles
depends on both genes
and the environment
•  Height is an example of a
phenotype strongly affected
by the environment.