Download Patterns of Inheritance

Patterns of Inheritance
Chapter 10
Blending Hypothesis of Inheritance


Trait
 A variation of a particular characteristic
Blending hypothesis (1800s)
 Early explanation of how offspring
inherit trait from both parents
 Example: if a red flower plant crossed
with a yellow flower, the offspring would
be orange
 Later discarded
Gregor Mendel
Austrian monk
 Father of Genetics (study of
heredity)
 Said parents pass on to their
offspring separate and distinct genes
 Studied 7 characteristics in pea plants


True breeding plants


A true plant will show the same physical
appearance generation after generation after
self-fertilization
Cross fertilization

The sperm from the pollen of one true flower
fertilizes the eggs in the flower of a different
plant
Mendel's Experiments


Cross-fertilized 2 true-breeding plants
each with contrasting traits (i.e. white and
purple flowers)
What color of flowers do you think the
offspring plants were?
Principle of Segregation


P generation
 Parental plants (purebred and true
breeding)
F1 generation ( F for filial “son”)
 Hybrid offspring


Hybrids
 The offspring of 2 different true-breeding
varieties
F2 generation
 When F1 self-fertilize or fertilize each other
Monohybrid Cross

Monohybrid cross
 Cross fertilization in which only one
physical characteristic is considered
 In Mendel's cross, all F1 were purple but
¼ of F2 were white
Gene Hypotheses #1
•
There are alternative forms of genes
which determine physical appearances


Allele is the term
Example: Flower color can be white or
purple
Gene Hypotheses #2
For each characteristic, an organism has
2 alleles for genes controlling the
physical appearances (one from each
parent)
•
•
•
If 2 alleles are the same= homozygous
If 2 alleles are different =heterozygous
Gene Hypotheses #3


Dominant alleles determine the physical
appearance in a heterozygous individual.
Recessive allele is the other allele that
does not affect the physical appearance


Capital letter represents dominant allele : P
Lower case letter represents recessive allele: p

Phenotype is the physical appearance




PP is purple
Pp is purple
pp is white
Genotype is the genetic makeup

Possible genotype are PP, Pp, pp.
Gene Hypotheses #4

The two alleles for a character segregate
(separate) during meiosis so that each
gamete carries only one allele for each
character, known as principle of
segregation.
Punnett Square

The alignment of combination of gametes
to form zygotes with pairs of alleles is
random


Like tossing a coin.
Punnett Square is a diagram that shows all
possible outcomes of a genetic cross.

Used to predict probabilities of outcomes if
you know the genotypes of the parents
The Testcross

Breeds an individual of unknown
genotype, but dominant phenotype
(purple) with a homozygous recessive
individual (white)


Appearance of F1 will reveal the genotype of
the mystery parent
If white flowers are produced, the unknown
parent must be heterozygous (Pp) and have a
recessive trait
Intermediate Dominance


Heterozygotes have a phenotype
intermediate between the phenotypes of
the two homozygote
Rules: (example: snapdragon flowers)




Capital/lower case letters not used
Instead, a C for “color” is paired with a
superscript R for “red” and W for “white”
CR CR is red and CW CW is white
CR CW is pink
Multiple alleles


Heterozygote express the distinct traits of
both alleles
Example: Human blood system
 A, B, AB, or O
 The letters are antigens found on the
surface of red blood cells
 Red blood cells may be coated with one
protein (A), the other (B), both (AB), or
neither (O)
 There are six possible genotype
combinations
Individuals whose red blood cells contain
protein A and lack protein B have type A
blood.
Those with protein B and lack protein A are
called type B.
Individuals with both protein A and protein
B are called type AB
Individuals with neither of the proteins is
called type O.
ABO blood type is a genetic example of
multiple alleles.
There are three alleles in the gene pool for
ABO blood type.
IA
IB
i
IA codes for protein A
IB codes for protein B
I codes for neither protein A nor protein B.
Within this multiple allele pool the gene
interactions illustrate both simple
dominance as well as co-dominance.
Remember each individual has only two
alleles for each trait even if there are
multiple alleles in the gene pool.
IAIA
both code for A type blood
IAi
Phenotype
Genotype
Protein on
RBC
(antigen)
Antibodies
in the blood
plasma
Type A
IA IA and
IA i
A
b
Type B
IB IB and IB
i
B
a
Type AB
IA IB
A and B
-------------
Type O
ii
-----------
a and b
ABO Blood System

Antibodies (proteins) also found in the
blood serum that attacks foreign antigens






Blood
Blood
Blood
Blood
A has antibody Anti-B
B has antibody Anti-A
AB has no antibody
O has Antibody Anti A and B
Blood O is the universal donor
Blood AB can receive any blood type
Rh Factor



Rh positive (Rh +) has protein in blood
Rh negative (Rh -) has no protein in blood
Rh+ is dominant
Blood Typing
Blood Type
Anti-a Sera
Anti-b Sera
A
Clumping
No clumping
B
No clumping
Clumping
AB
Clumping
Clumping
O
No clumping
No clumping
Polygenic Inheritance


Combined effect of 2+ genes on a single trait
Alleles A, B, C, D are all inherited separately, but
contribute to height.
 Capital letters contribute to a “unit” of tallness
 Lower case letter contribute to a “unit” of
shortness
 More capital letters is tallest (AABBCCDD) and
all lower case letters is the shortest
(aabbccdd)
 Blend of upper/lower case letters is a medium
(AaBbCcDd)
Polygenic Traits






Height
Weight
Eye Color
Intelligence
Skin Color
Many forms of behavior
Genetic Linkage


Gene locus
 Same location on a homologous
chromosome where alleles reside
The tendency for the alleles on one
chromosome to be inherited together is
genetic linkage
 The closer the genes are, the greater the
genetic linkage
 During crossing over, the chances of alleles
staying together increases the closer they
are.
Sex-linked genes

The eggs contain a single X chromosome
and sperm contain either an X or a Y


Sex of the offspring depends on whether the
sperm that fertilizes the egg has an X or a Y
Any gene located on a sex chromosome
(X) is called a sex-linked gene

Most are found on the X (2,000) and few on
the Y (24)
Sex-linked traits



Written as a XRXr for heterozygous.
Y chromosome carries no allele and the
phenotype is dependant upon the
woman’s allele
Therefore, males carry one allele for a
sex-linked trait.
Sex-linked disorders


Red-green blindness
Hemophilia (inability of blood to clot)