1. Two or More Traits: Dihybrid Crosses
Mendelian Genetics
Two or More Traits: Dihybrid Crosses
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2. Summary of Mendel’s laws
Mendelian Genetics
11/9/2018
Summary of Mendel’s laws
LAW
PARENT CROSS
OFFSPRING
DOMINANCE
TT x tt tall x short
100% Tt tall
SEGREGATION
Tt x Tt tall x tall
75% tall 25% short
INDEPENDENT ASSORTMENT
RrGg x RrGg round & green x round & green
9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods
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Mendelian Genetics
11/9/2018
Dihybrid Cross
A breeding experiment that tracks the inheritance of two traits.
Mendel’s “Law of Independent Assortment”
a. Each pair of alleles segregates independently during gamete formation
b. Formula: 2n (n = # of heterozygotes)
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4. All possible gamete combinations
Mendelian Genetics
11/9/2018
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round r wrinkled Y yellow y green
RrYy x RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
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Mendelian Genetics
11/9/2018
Dihybrid Cross RY Ry rY ry RY Ry rY ry
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Mendelian Genetics
11/9/2018
Dihybrid Cross RY Ry rY ry
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic ratio
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy
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Mendelian Genetics
11/9/2018
Dihybrid Cross
Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1
9:3:3:1
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8. Non-Mendelian Genetics

9. OBJECTIVE
SWL the genetic patterns of inheritance and how they can be displayed. They will also learn what disorders are dominant and recessive.

10. Chromosomal theory of heredity – genes are located on chromosomes
Practical application – Mendel’s laws only sort of apply some of the time.

11. Gene Linkage Not all genes will go through independent assortment.
Genes on the same chromosome are called linked genes

12. TH Morgan worked with fruit flies in early 1900’s
Instead of 1:1:1:1 like he expected in one of his many crosses, most flies were grey with large wings OR black with small wings.
Morgan concluded that wing size and body color were linked.
Linkage groups – groups of genes that are always inherited together

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14. Crossing-Over
HOWEVER – some genes aren’t always inherited together in fruit flies
Morgan proposed “crossing-over” to explain why this happened

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Using the frequency of crossing over allows you to construct a “linkage map” showing how far apart genes are
More frequent crossing-over – genes are farther apart (dice and letters activity)
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18. Other Non-Mendelian genetics

19. OBJECTIVE
SWL the difference between various complex disorders and how they can be analyzed and adapted for.

20. Sex linkage
In 1905, Nettie Stevens found that not all chromosomes have a corresponding homologous chromosome.
Sex chromosomes – look like a mismatch
Autosomes – same in both female and males

21. Sex determination
XX is female (in humans)
XY is a mismatch to be male (in humans)

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23. Sex-linked genes
Exist on one of the sex chromosomes, either X or Y (usually X)
i.e. eye color in fruit flies, color-blindness and hemophilia in humans
Because males only have one x, they always display the trait

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25. Gene Interactions
Genes produce proteins – dominant genes produced much more protein than recessive ones.
This is NOT always the case.

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1. Incomplete Dominance 
In incomplete dominance, the heterozygous genotype results in a phenotype between the two homozygous phenotypes.
i.e. white paint + red paint = pink
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A cross between red and white snapdragons produces pink-colored flowers.
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.
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2. Codominance 
In codominance, both alleles contribute to the phenotype.
In cattle, red hair is codominant with white hair. Cattle who are roan have both the red and white allele – they do not blend, they appear separately.
VERY SIMILAR to incomplete dominance!!
Generally goes along with multiple allelic traits
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3. Multiple Allelic Traits 
Genes that are controlled by more than two alleles are said to have multiple alleles.
An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.
i.e. Blood types in humans and rabbits’ coat color
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Different combinations of alleles result in the colors shown here.
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles
ch = Himalayan; color in certain parts of the
body; dominant to
c allele
c = albino; no color;
recessive to all other
alleles
Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc.
AIbino: cc
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
Chinchilla: cchch, cchcch, or cchc
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4. Polygenic Traits  
Traits controlled by two or more genes are said to be polygenic traits.
Skin color, diabetes, height, cleft palate, cancer, etc in humans are polygenic traits controlled by at least ____ different genes.
????
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33. 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc
Fraction of progeny
6/64
1/64

34. Answer these in your notes!
Page 303 data analysis graph questions 1-2.
Define and put vocabulary words (pg. 302) in your notebooks.

35. Mutations – when something goes wrong
Some good, some bad
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Kinds of Mutations
Kinds of Mutations
Mutations are changes in the genetic material.
Mutations that produce changes in a single gene are known as gene mutations
(We’ll get to these in a few weeks. They have the same names, but occur WITHIN a gene not a whole chromosome.)
Mutations that produce changes in whole chromosomes are known as chromosomal mutations.
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Kinds of Mutations
Chromosomal Mutations 
Chromosomal mutations involve changes in the number or structure of chromosomes.
Chromosomal mutations include deletions, duplications, inversions, and translocations.
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39. Cri du chat – chromosome 5
Kinds of Mutations
Deletions involve the loss of all or part of a chromosome.
Chromosomal mutations involve changes in whole chromosomes.
Cri du chat – chromosome 5
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Kinds of Mutations
Duplications produce extra copies of parts of a chromosome.
Chromosomal mutations involve changes in whole chromosomes.
Angelman syndrome – Chromosome 15
Fragile X – X chromosome
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Kinds of Mutations
Inversions reverse the direction of parts of chromosomes.
Chromosomal mutations involve changes in whole chromosomes.
5% of Downs syndrome cases
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42. Translocations occurs when part of one chromosome breaks off and attaches to another.
Happens frequently and with fewer lethal consequences in plants and bacteria
Chromosomal mutations involve changes in whole chromosomes.
Most commonly (1:1000) between chromosome 13 and 14
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43. Significance of Mutations
Many mutations have little or no effect on gene expression.
Some mutations are the cause of genetic disorders.
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44. Nondisjunction – homologous chromosomes did NOT separate during meiosis, results in several human disorders
i.e. Turner syndrome, trisomy 21, Klinefelter’s syndrome
Polyploidy - an organism has extra sets of chromosomes. (Most plants are at least tetraploid)
i.e. triploid has three sets, tetraploid has four
Aneuploidy is a condition when there is one too few or too many chromosomes.
i.e. Trisomy 21 (Downs syndrome) is 3 of chromosome 21; poly-X females have 3+ X chromosomes

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