1. 3.4 Inheritance
The patterns that genes and the phenotypes they generate can be mapped using pedigree charts. The image show a small section of a pedigree chart that maps the inheritance of hair colour in an extended family over several generations. Analysis of pedigree charts enables us to see the nature of the inheritance; controlled by dominant or recessive alleles? linked to the sex chromosomes? controlled by multiple genes or a single gene?

2. 3.4 Inheritance
Essential idea: The inheritance of genes follows patterns.

3. WARNING: Includes a scientific scandal
3.4.U1 Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
Mendel’s principles of inheritance
Johann Gregor Mendel
( )
Learn about Mendel and his work by using the weblinks
Gregor Mendel: Great Minds by SciShow
Because of his work with pea plants Mendel is considered the
He planted 1000s of seeds per trial and carried out many trials to be sure of his results.
His published work (1865) is now considered important, but at the time was ignored for 30 years.
WARNING: Includes a scientific scandal
Biologica: Mendel’s Peas
Gregor Mendel and pea plants

4. Definitions This image shows a pair of homologous chromosomes.
Name and annotate the labeled features.

5. The alleles present at a gene locus maybe similar or different.
3.4.U3 The two alleles of each gene separate into different haploid daughter nuclei during meiosis. AND 3.4.U2 Gametes are haploid so contain only one allele of each gene. AND 3.4.U4 Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
Because fertilization involves the fusion of gametes the number of chromosomes is doubled. The diploid organism also now contains two alleles for each gene.
Meiosis halves the chromosomes present in gametes and reduces the number of alleles of each gene from two to one.
The alleles present at a gene locus maybe similar or different.

6. 3.4.A1 Inheritance of ABO blood groups.
The ABO blood type classification system uses the presence or absence of certain antigen on red blood cells to categorize blood into four types.
Distinct molecules called agglutinogens (a type of antigen) are attached to the surface of red blood cells. There are two different types of agglutinogens, type "A" and type "B”.

7. (immunoglobulins) are .
3.4.A1 Inheritance of ABO blood groups.
(immunoglobulins) are
The immune system recognizes 'foreign' antigens and produces antibodies in response - so if you are
Blood type O is known as the , as it has against which the recipient immune system can react. Type AB is the , as it has
A Nobel breakthrough in medicine.
Images and more information from:
Blood typing game from Nobel.org:

8. (characteristic expressed)
3.4.A1 Inheritance of ABO blood groups.
The ABO blood type is controlled by a single gene, the ABO gene. This gene has three different alleles:
i O allele (no anitgen is produced)
IA A allele (type “A” anitgen is produced)
IB B allele (type “B” anitgen is produced) IA
Allele variant
Gene (lower case for ‘recessive’ alleles)
Diploid cells possess two alleles therefore the possible genotype and phenotype combinations are:
Genotype
(allele combination)
Antigen production
Phenotype
(characteristic expressed) ii
Blood type
IAIA and IAi
IBIB and IBi
IAIB

9. 3.4.U5 Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
Dominant alleles have the same effect on the phenotype whether it is present in the homozygous or heterozygous state
Type “A” allele present and blood type is A therefore the type
IAi
Type “O” allele present and blood type is not O therefore the type
Recessive alleles only have an effect on the phenotype when present in the homozygous state
Codominant alleles are pairs of different alleles that both affect the phenotype when present in a heterozygote
IAIB
Type “A” and “B” alleles are present and blood type is AB therefore type

10. 3.4.U5 Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
Dominant alleles have the same effect on the phenotype whether it is present in the homozygous or heterozygous state
Type “A” allele present and blood type is A therefore the type “A” allele is dominant to type “O”
Reality check: dominant and recessive inheritance are useful concepts for predicting the probability of inheriting certain phenotypes, especially genetic disorders. ABO Blood type is unusual as only one gene is involved in the expression of the phenotype. In most cases multiple genes contribute and/or interact to produce a trait. Whether an allele is regarded as dominant or recessive is relative and depends on the particulars of the proteins coded for.
IAi
Type “O” allele present and blood type is not O therefore the type “O” allele is recessive to type “A”
Recessive alleles only have an effect on the phenotype when present in the homozygous state
Codominant alleles are pairs of different alleles that both affect the phenotype when present in a heterozygote
IAIB
Type “A” and “B” alleles are present and blood type is AB therefore type “A”and “B” alleles are codominant

11. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Explain this
Mendel crossed some yellow peas with some yellow peas. Most offspring were yellow but some were green!
Mendel from:

12. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Segregation
“alleles of each gene separate into different gametes when the individual produces gametes”
The yellow parent peas must be The yellow phenotype is expressed.
Through meiosis and fertilization, some offspring peas are – they express a green color.
Mendel did not know about DNA, chromosomes or meiosis.
Through his experiments he did work out that ‘heritable factors’ (genes) were passed on and that these could have different versions (alleles).
Mendel from:

13. Segregation F0 F1 Y y Y y Y or y Y or y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Segregation
“alleles of each gene separate into different gametes when the individual produces gametes” F0
Key to alleles:
Y = yellow
y = green
Genotype:
Y y
Y y
Alleles segregate during meiosis (anaphase I) and end up in different haploid gametes.
Y or y
Y or y
Gametes:
Punnet Grid:
gametes
Simplified notation of using upper case for dominant and lower case for recessive is acceptable in the case of two alleles without co-dominance. F1
Genotypes:
Phenotypes:
Phenotype ratio:
Mendel from:

14. Monohybrid Cross F0 F1 Crossing a single trait. Y y Y y Y or y Y or y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
Crossing a single trait. F0
Key to alleles:
Y = yellow
y = green
Genotype:
Y y
Y y
Alleles segregate during meiosis (anaphase I) and end up in different haploid gametes.
Fertilization results in diploid zygotes.
A punnett grid can be used to deduce the potential outcomes of the cross and to calculate the expected ratio of phenotypes in the next generation (F1).
Y or y
Y or y
Gametes:
Punnet Grid:
gametes F1
Genotypes:
Phenotypes:
Phenotype ratio:
Mendel from:

15. Monohybrid Cross F0 F1 Y y YY Yy yy Y y Y y Y or y Y or y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
Crossing a single trait. F0
Key to alleles:
Y = yellow
y = green
Genotype:
Y y
Y y
Alleles segregate during meiosis (anaphase I) and end up in different haploid gametes.
Fertilisation results in diploid zygotes.
A punnet grid can be used to deduce the potential outcomes of the cross and to calculate the expected ratio of phenotypes in the next generation (F1).
Y or y
Y or y
Gametes:
Punnet Grid:
gametes Y y YY Yy yy F1
Genotypes:
Phenotypes:
Phenotype ratio:
Mendel from:

16. Monohybrid Cross F0 F1 Y y YY Yy yy Y y Y y Y or y Y or y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
Crossing a single trait. F0
Key to alleles:
Y = yellow
y = green
Genotype:
Y y
Y y
Alleles segregate during meiosis (anaphase I) and end up in different haploid gametes.
Fertilisation results in diploid zygotes.
A punnet grid can be used to deduce the potential outcomes of the cross and to calculate the expected ratio of phenotypes in the next generation (F1).
Ratios are written in the simplest mathematical form.
Y or y
Y or y
Gametes:
Punnet Grid:
gametes Y y YY Yy yy F1
Genotypes: YY Yy Yy yy
Phenotypes:
Phenotype ratio:
Mendel from:

17. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
What is the expected ratio of phenotypes in this monohybrid cross? F0
Key to alleles:
Y = yellow
y = green
Phenotype:
Genotype:
Homozygous recessive
Homozygous recessive
Punnet Grid:
gametes F1
Genotypes:
Phenotypes:
Phenotype ratio:

18. Monohybrid Cross F0 F1 y yy y y y y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
What is the expected ratio of phenotypes in this monohybrid cross? F0
Key to alleles:
Y = yellow
y = green
Phenotype:
Genotype:
y y
y y
Homozygous recessive
Homozygous recessive
Punnet Grid:
gametes y yy yy yy yy yy F1
Genotypes:
Phenotypes:
Phenotype ratio:

19. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
What is the expected ratio of phenotypes in this monohybrid cross? F0
Key to alleles:
Y = yellow
y = green
Phenotype:
Genotype:
Homozygous recessive
Heterozygous
Punnet Grid:
gametes F1
Genotypes:
Phenotypes:
Phenotype ratio:

20. Monohybrid Cross F0 F1 Y y Yy yy y y Y y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
What is the expected ratio of phenotypes in this monohybrid cross? F0
Key to alleles:
Y = yellow
y = green
Phenotype:
Genotype:
y y
Y y
Homozygous recessive
Heterozygous
Punnet Grid:
gametes Y y Yy yy F1
Genotypes: Yy Yy yy yy
Phenotypes:
Phenotype ratio:

21. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
What is the expected ratio of phenotypes in this monohybrid cross? F0
Key to alleles:
Y = yellow
y = green
Phenotype:
Genotype:
Homozygous dominant
Heterozygous
Punnet Grid:
gametes F1
Genotypes:
Phenotypes:
Phenotype ratio:

22. Monohybrid Cross F0 F1 Y y YY Yy Y Y Y y
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Monohybrid Cross
What is the expected ratio of phenotypes in this monohybrid cross? F0
Key to alleles:
Y = yellow
y = green
Phenotype:
Genotype:
Y Y
Y y
Homozygous dominant
Heterozygous
Punnet Grid:
gametes Y y YY Yy F1
Genotypes: YY YY Yy Yy
Phenotypes:
Phenotype ratio:

23. Steps to figuring out Monohybrid Crosses:
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Steps to figuring out Monohybrid Crosses:
Write down all information given
Write down phenotypes & genotypes of parents
Figure out gametes
Draw punnett grid & fill in
Determine the genotypic and phenotypic ratio for offspring

24. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Practice Problem:
In guinea pigs, black fur is dominant to white fur. Cross a heterozygous black guinea pig with a white guinea pig. Give the genotypes and phenotypes of the offspring. Black is dominant to white.
Solution: Bb (Black) x bb (white)
Genotypic ratio:
Phenotypic ratio:
*Do on board

25. Ratios for a Monohybrid Cross:
3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Ratios for a Monohybrid Cross:
Anytime you cross 2 heterozygotes you always get:
Genotypic ratio = 1 AA: 2 Aa: 1 aa Phenotypic ratio = 3 dominant: 1 recessive
Anytime you cross a homozygote with a heterozygote you get:
Genotypic ratio = 1 homozygous: 1 heterozygote
These ratios can also be expressed as percentages:
3:1  75% tall; 25% short
1:2:1  25% TT; 50%Tt; 25% tt

26. Exceptions to Mendel’s Laws:
Many traits are not just controlled by one pair of genes that are dominant or recessive
Many traits are controlled by many pairs of genes cooperating to control the expression of a single trait

27. Hybrid: CR CW = Pink (in between)
Incomplete dominance – alleles are not always completely dominant or recessive. Result is an
Ex: Snapdragons
CR = red, CW = white
CR CR = Red, CW CW = White
Hybrid: CR CW = Pink (in between)

29. Codominance – the (ex. roan coat color in horses and cows, human blood types)
Human blood type: Both IA and IB are dominant to iO and are codominant to each other

30. Example – Roan coat in cattle
Alleles: R – red, W – white
RR = Red, WW = White, Hybrid (RW) – Roan coat: equal amounts of red and white hair

31. Multiple alleles – when. within a population (ex
Multiple alleles – when within a population (ex. Coat color in rabbits, eye color in fruit flies, human blood types)
Any one individual can have only up to two alleles for each gene.
There are 3 different alleles for human blood types (multiple alleles)  IA, IB, i

32. 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

33. 3.4.S1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
Practice Problem:
A woman with type A blood marries a man with type AB blood. What blood types could you expect to find among their children?
What is the woman’s genotype if the couple produces children with type B blood?
woman IAIA or IAiO
man IAIB
IA IB
IA IB IA IA iO

34. Phenotype does not always reveal the genotype
3.4.S2 Comparison of predicted and actual outcomes of genetic crosses using real data.
Phenotype does not always reveal the genotype
For example, an individual that is heterozygous has the dominant phenotype but will have both the dominant and recessive allele.
To determine genotype a may be done.
In a test cross, an individual of
Key to alleles:
R = Red flower
r = white
RR or Rr?

35. 3.4.S2 Comparison of predicted and actual outcomes of genetic crosses using real data.
Test Cross
Used to determine the genotype of an unknown individual.
The unknown is crossed with a known homozygous recessive. F0
Key to alleles:
R = Red flower
r = white
Phenotype:
Genotype:
R ?
r r
unknown
Homozygous recessive
Possible outcomes: F1
Phenotypes:
Unknown parent = RR
Unknown parent = Rr
gametes
gametes

36. occurs when a gene is (usually the X  ).
3.4.U7 Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.
occurs when a gene is (usually the X  ).
Recessive sex-linked traits are Why?
Females have 2 X chromosomes
Possible genotypes: XHXH, XHXh, XhXh
Phenotype: dominant dominant recessive
Females can either be homozygous or heterozygous for sex-linked traits.
Males have only 1 X chromosome
Possible genotypes: XHY, XhY
Phenotype: dominant recessive

37. Sex Linkage What number do you see?
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Sex Linkage
X and Y chromosomes are non-homologous.
What number do you see?
Chromosome images from Wikipedia:

38. Sex Linkage What number do you see? 5 = normal vision
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Sex Linkage
X and Y chromosomes are non-homologous.
What number do you see?
5 = normal vision
2 = red/green color blindness
Chromosome images from Wikipedia:

39. Sex Linkage XN XN XN Y Xn Xn Xn Y XN Xn Xq28
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Sex Linkage
X and Y chromosomes are non-homologous.
How is colour-blindness inherited?
The red-green gene is carried at locus Xq28.
This locus is in the non-homologous region, so there is no corresponding gene on the Y chromosome.
Normal vision is dominant over color-blindness.
XN XN
XN Y
no allele carried, none written
Key to alleles:
N = normal vision
n = red/green color
blindness
Normal female
Normal male
Xq28
Xn Xn
Xn Y
Affected female
Affected male
Human females can be homozygous or heterozygous with respect to sex-linked genes. Heterozygous females are carriers.
XN Xn
Carrier female

40. Sex Linkage XN Xn XN Y F0 F1 X Key to alleles: N = normal vision
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Sex Linkage
Key to alleles:
N = normal vision
n = red/green color
blindness
What is the chance of a color-blind child in the cross between a normal male and a carrier mother? F0
XN Xn
XN Y
Genotype: X
Phenotype:
Carrier female
Normal male
Punnett Grid: F1

41. Red-Green Color Blindness
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Red-Green Color Blindness
How does it work?
The OPN1MW and OPN1LW genes are found at locus Xq28.
They are responsible for producing photoreceptive pigments in the cone cells in the eye. If one of these genes is a mutant, the pigments are not produced properly and the eye cannot distinguish between green (medium) wavelengths and red (long) wavelengths in the visible spectrum.
Xq28
Because the Xq28 gene is in a non-homologous region when compared to the Y chromosome, red-green color blindness is known as a The male has no allele on the Y chromosome to combat a recessive faulty allele on the X chromosome.
Chromosome images from Wikipedia:

42. Hemophilia XH XH XH Y Xh Xh Xh Y XH Xh Another sex-linked disorder.
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Hemophilia
Another sex-linked disorder.
Blood clotting is an example of a metabolic pathway – a series of enzyme-controlled biochemical reactions.
It requires globular proteins called
A in hemophiliacs results in one of these factors not being produced. Therefore, the clotting response to injury does not work and the patient can bleed to death.
XH XH
XH Y
no allele carried, none written
Normal female
Normal male
Key to alleles:
XH = healthy clotting factors
Xh = no clotting factor
Xh Xh
Xh Y
Affected female
Affected male
Human females can be homozygous or heterozygous with respect to sex-linked genes.
Heterozygous females are carriers.
XH Xh
Carrier female
Chromosome images from Wikipedia:

43. Hemophilia Read/ research/ review:
3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
Hemophilia
results from a lack of clotting factors. These are globular proteins, which act as enzymes in the clotting pathway.
Read/ research/ review:
How can gene transfer be used to treat hemophiliacs?
Gene Therapy Explained:
What is the relevance of “the genetic code is universal” in this process?
Chromosome images from Wikipedia:

44. 3.4.A2 Red-green color blindness and hemophilia as examples of sex-linked inheritance.
results from a lack of clotting factors. These are globular proteins, which act as enzymes in the clotting pathway.