Download Sex-linked Traits

Genetics
Gregor Mendel: Father of Genetics

Genetics: study of
heredity

Heredity: passing
traits from parent to
offspring

Used peas to study
heredity
Why Peas?



1. Several Traits
2. Control mating
3. Small, grow easily, and matures
quickly
Alleles: Different versions of a gene
Ex: White and Purple Flowers
Mendel’s Experiment

Monohybrid Cross: One pair of contrasting
traits.


Ex: Mating a plant with purple flowers with white
flowers
True-breeding: All offspring would have
only one form of the trait

Ex: Purple-flowering plant only has purpleflowered offspring
Mendel’s Experiments
Vocabulary

Dominant: Expressed in appearance (A)

Recessive: Not noticeable in appearance (a)

Homozygous: AA or aa

Heterozygous: Aa
More Vocabulary

Genotype: Genetic make up (ex: Pp)

Phenotype: Appearance (ex: Purple flowers)
Laws of Heredity

Law of Segregation

The two alleles for a trait separate when gametes are
formed

Law of Independent Assortment

Alleles of different genes separate independently of
each other during meiosis
Law of Segregation

Separation of alleles into
separate gametes

50% chance that one
allele will go into a
gamete
Law of Independent Assortment
Punnett Square
Ratios
Test Cross
Dihybrid Cross: Cross that involve two traits
Incomplete Dominance
and
Codominance
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Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
 Example: snapdragons (flower)
 red (RR) x white (rr)
r
r



RR = red flower R
rr = white flower
R
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Incomplete Dominance
r
r
R Rr
Rr
R Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
19
Incomplete Dominance
20
Codominance
Two alleles are expressed (multiple alleles)
in heterozygous individuals.
 Example: blood type





1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Codominance Problem
Example:homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
1/2 = IAIB
1/2 = IBi
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
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Sex-linked Traits
Traits (genes) located on the sex
chromosomes
 Sex chromosomes are X and Y
 XX genotype for females
 XY genotype for males
 Many sex-linked traits carried on X
chromosome

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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
25
Sex-linked Trait Problem


Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XR Y






x
XrXr
Remember: the Y chromosome in males does
not carry traits.
RR = red eyed
Xr
Xr
Rr = red eyed
rr = white eyed
XY = male
XR
XX = female
Y
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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Female Carriers
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