chapter 12 – patterns of inheritance what is inheritance? why study inheritance? what is the...
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Chapter 12 – Patterns of Inheritance
What is inheritance?
Why study inheritance?
What is the relationship between genes, alleles, phenotype and genotype?
What is genetic linkage?
How can inheritance be predictive? (why do we bother with Punnett squares?
What is inheritance?
The genetic transmission of characteristics from parents to offspring
Geneticists study inheritance
Genes are inherited
Why study inheritance?
Many diseases involve inheritance
Why study inheritance?
Genetics benefits medical advances
What is the relationship between genes, alleles, genotype and phenotype?
What is the relationship between genes, alleles, genotype and phenotype?
Alleles are differences (variation) in a gene within a population of a species
Genes determine phenotype (physical characteristic) of a structure, molecule, etc.
Why are there alleles?
Point mutations in the wild type gene result in new alleles
Alleles, genotype and phenotype
1.Diploid organisms have 2 alleles for every gene
2.For each gene, the 2 alleles may be identical or they may be differenta)Homozygous – possessing 2 identical alleles
for a gene
b)Heterozygous – possessing 2 different alleles for a gene
What is the relationship between genes, alleles, genotype and phenotype?
1.The type of interaction between the pair of inherited alleles for a gene determines the phenotypea)Dominant allele – expressed, regardless of the
other allele present
b)Recessive allele – not expressed in the presence of a dominant allel
What is a genotype?
A description of the alleles present for a gene
1.Homozygous dominant
2.Homozygous recessive
3.Heterozygous
How is genotype symbolized?
Capital letter = dominant allele
Lower case letter = recessive allele
What is the relationship between genes, alleles, genotype and phenotype?
Tongue rolling gene has 2 different alleles
Rr Rr
Rr Rr rr RR
What is the relationship between genes, alleles, genotype and phenotype?
Not all genes/alleles display dominant/recessive interactions
1.Codominance – both alleles for a gene are expressed (example: two forms of an enzyme)
2. Incomplete dominance – heterozygous phenotype is different from either the dominant or recessive phenotype
3.Many traits influenced by more than one gene
Red flowers and thorns are dominant to white flowers and no thorns in roses. What would be the phenotype of a rose heterozygous for both traits?
1.RrTt
2.Red, thorny
3.RRTT
4.White, thornless
5.rrtt
Red flowers and thorns are dominant to white flowers and no thorns in roses. Which of the following can be a genotype for a red, thorny rose?
1.RT
2.RRTT
3.RrTT
4.RrTt
5.2, 3 & 4
What is genetic linkage?
Genes located close together on the same chromosome are genetically linked (i.e they tend to get inherited together)
Mapping is based on genetic linkage
Mapping – identifying the location of a gene on a chromosome
Some genes are more likely to stay together (during meiosis), other genes are not
11 22 33 44
5566 77
Genetic linkage vs. independent assortment
Which genes are most likely to be inherited together?
11 22 33 44
5566 77
1a1a 2a2a 3a3a 4a4a
5a5a 6a6a 7a7a
Which genes are most likely to be inherited together?
1)2 & 2a
2)1 & 5
3)1a&2a
4)5&7
5)1a & 4a
6)Responses 3, 4 & 5 are all correct
1 2 3 4
56 7
1a 2a 3a 4a
5a 6a 7a
Independent assortment of chromosomes “shuffles” maternally inherited and paternally inherited genes during meiosis
1 2 3 4
56 7
1-7 = genes materally inherited (maternal alleles)
1a-7a = genes paternally inherited (paternal alleles)
5a 6a7a
1a 2a 3a 4a 1 2 43
1a 2a 3a 4a 56 7
5a 6a7a
Crossing over breaks genetic linkage
1a2a 3a 4a
24
3
1
1a2 43
1 2a 3a 4a
How can inheritance be predictive?
How can one know if two genes are located on the same chromosome or on different chromosomes?
A
A
B
BA
A B
B
Mapping genes and genetic crosses
Cross individuals different in two genetic traits:
AABB x aabb
II. The foundation of genetics
A. Gregor Mendel (1860s)
PPPP pppp
PpPp
genotypes
(phenotype) (phenotype)
Pp x PpPp x PpPP
pp
PP ppPossible Possible gametesgametes
PP pp PP pp
Homologous pairs are separated in meiosis
meiosismeiosis
Pp x PpPp x PpPP
pp
PP pp
PPPP
PpPp
PpPp
pppp
Possible Possible gametesgametes
Geneticists utilize dihybrid crosses
Dihybrid crosses involve two genes
AABB x aabb
A
A
B
B
Assuming independent assortment
Genetic linkage
A
A B
B A
AB
Boror
Are the two genes for eye color located on the same chromosome?
AABB x aabb
F1: all AaBb
Self the F1
AaBb x AaBb
What types of gametes are produced?
Gametes, assuming independent assortment
a
A
b
B
a
A
B
b
a
A B
b
or
Ab
aB
AB
ab
AaBb
Gametes, assuming independent assortment
a
A
b
B
a
A
B
b
a
A B
b
or
Ab
aB
AB
ab
AaBb
Independent assortment
Gametes, assuming complete linkage
a
AB
b
AaBb
a b
A BAB
ab
Complete linkage
AaBb x AaBb
ABAB
ABAB
abab
abab
AABB
AaBb
AaBb
aabbaa
AA BB
bb
Genetic linkage can be broken
Crossing over can break linkage
Gametes produced:
AB
Ab
aB
ab
Gametes, assuming independent assortment
a
A
b
B
a
A
B
b
a
A B
b
or
Ab
aB
AB
ab
AaBb 25%
25%
25%
25%
Gamete frequency – linked genes
a
AB
b
No crossing over
AB
ab
With crossing over AB
ab
Ab
aB
50%
50%
40%
40%
10%
10%
Linkage between genes varies with distance between genes
A B C D
a b c d
Linkage vs. independent assortment
For two single gene traits, if perform a dihybrid cross of heterozygotes:Independent assortment gives 4 phenotypes
in offspring
Complete linkage gives <4 phenotypes
Partial linkage gives 4 phenotypes in a different ratio than independent assortment
Genetics & disease
Most disease genes are autosomal & recessive (Most mutations function as recessive alleles to “normal” functioning alleles)
Each person is a “carrier” of several recessive disease related alleles
Sex-linked traits
• Genes located on X-chromosome
• Males possess only 1 allele for sex-linked traits