1. Inheritance of genetic traits
Chapter 7

2. Making a Pedigree
A pedigree is a graphic representation of genetic inheritance
It is a diagram made up of a set of symbols that identify males and females, individuals affected by the trait being studied, and family relationships

3. *Carrier: A half-shaded circle or square
* Female: circle
*Male: square
*Affected trait: completely filled in
*Death: / line
*Mating: connected
*Siblings: below
Male
Parents
Siblings
Female
Affected male
Known heterozygotes for recessive allele
Affected female
Death
Mating

4. Each row shows a generation with most recent at the bottom
*Generations are marked with a Roman Numeral
*Individuals are in order based on order of birth I
Female
Male 1 2 II 1 2 3 4 5
III 1 2 3 4 IV 1 2 3 4 5

5. Simple Recessive Heredity
Most genetic disorders are caused by recessive alleles

6. 1. Cystic Fibrosis
Cystic fibrosis (CF) is a fairly common genetic disorder among white Americans
About 1 in 28 white Americans carries the recessive allele, & 1 in 2500 children born to white Americans inherits the disorder.
Due to a defective protein in the plasma membrane, cystic fibrosis results in the formation and accumulation of thick mucus in the lungs and digestive tract

7. 2. Tay Sachs Disease
Tay-Sachs (tay saks) disease is a recessive disorder of the central nervous system among the Jewish people in the U.S. & eastern European descent
A recessive allele results in the absence of an enzyme that normally breaks down a lipid produced and stored in tissues of the central nervous system
Because this lipid fails to break down properly, it accumulates in the cells

8. Typical pedigree for Tay-Sachs
Symptoms are not
usually visual until
around 48 months. Child
gradually becomes blind
and helpless, develops
seizures & eventually
paralyzed.
The child dies by the age
of 3 or 4
No treatment or cure.
Can be detected by
amniocenteses 1 2 II 1 2 3 4
III 1 2 3 IV 1

9. 3. PKU
PKU or Phenylketonuria (fen ul kee tun YOO ree uh), is a recessive disorder that results from the absence of an enzyme that converts one amino acid, phenylalanine, to a different amino acid, tyrosine.
Because phenylalanine cannot be broken down, it and its by-products accumulate in the body and result in severe damage to the central nervous system

10. Phenylketonurics: Contains Phenylalanine
Phenylketonuria
A PKU test is normally performed on all infants a few days after birth
Infants affected by PKU are given a diet that is low in phenylalanine until their brains are fully developed
Phenylketonurics: Contains Phenylalanine

11. Simple Dominant
Many traits are inherited just as the rule of dominance predicts
Remember that in Mendelian inheritance, a single dominant allele inherited from one parent is all that is needed for a person to show the dominant trait

13. Some examples of Simple Dominance
A cleft chin
Widow’s peak hairline
Hitchhiker’s thumb
Almond shaped eyes
Thick lips
Presence of hair on the middle section of your fingers

14. Huntington’s Disease
Huntington’s disease is a lethal genetic disorder caused by a rare dominant allele
It results in a breakdown of certain areas of the brain
Usually, dominant allele with such severe effects would result in death before the affected individual could have children and pass the allele on to the next generation
But because the onset of Huntington’s disease usually occurs between the ages of 30 and 50, an individual may already have had children before knowing whether he or she is affected.

15. Typical Pedigree of Huntington’s Disease
leads to progressive
degeneration of brain
cells, which causes
muscle spasms,
personality disorders &
death in years
from onset 1 2 II 1 2 3 4 5
III 1 2 3 4 5

16. When inheritance follows different rules

17. Incomplete Dominance
When traits are inherited in an incomplete dominance pattern, the presence of a 3rd phenotype appears.
For example, if a homozygous red-flowered snapdragon plant (RR) is crossed with a homozygous white-flowered snapdragon plant (WW), all of the F1 offspring will have pink flowers

18. Appearance of a 3rd Phenotype
Red
White
All pink
Red (RR)
Pink (RR’)
White (R’R’)
Pink (RR’)
All pink flowers
1 red: 2 pink: 1 white

19. Why a 3rd Phenotype?
The new phenotype occurs because the flowers contain enzymes that control pigment production
The R allele codes for an enzyme that produces a red pigment. The R’ allele codes for a defective enzyme that makes no pigment.
Because the heterozygote has only one copy of the R allele, its flowers appear pink because they produce only half the amount of red pigment that red homozygote flowers produce.

20. Codominance: expression of both alleles
Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygous individuals.
In codominance, both alleles are expressed equally

21. Sickle Cell Anemia
In an individual who is homozygous for the sickle-cell allele, the oxygen-carrying protein hemoglobin differs by one amino acid from normal hemoglobin
This defective hemoglobin forms crystal-like structures that change the shape of the red blood cells. Normal red blood cells are disc-shaped, but abnormal red blood cells are shaped like a sickle, or half-moon

22. HbAHbA individuals are normal; HbS HbS have sickle-cell trait.
With sickle-cell disease, red blood cells are irregular in shape (sickle-shaped) rather than biconcave, due to abnormal hemoglobin that the cells contain.
Symptoms results in poor circulation, anemia, low resistance to infection, hemorrhaging, damage to organs, jaundice, and pain of abdomen and joints.
Bone marrow transplants pose high risks; other research focuses on fetal hemoglobin, etc

23. Sickle-cell anemia
Abnormally shaped blood cells, slow blood flow, block small vessels, and result in tissue damage and pain.
Normal red blood cell
Sickle cell

25. Multiple Alleles:
Although each trait has only two alleles in the patterns of heredity you have studied thus far, it is common for more than two alleles to control a trait in a population
Traits controlled by more than two alleles have multiple alleles

26. Multiple alleles: govern blood type
Mendel’s laws of heredity also can be applied to traits that have more than two alleles.
The A-B-O blood group is a classic example of a single gene that has multiple alleles in humans.
Determining blood type is necessary before a person can receive a blood transfusion because the red blood cells of incompatible blood types could clump together, causing death

27. Multiple Alleles Govern Blood Type
Human Blood Types
Genotypes
Surface Molecules
Phenotypes A A
lA lA or lAli
lB lB or lBi B B
lA lB
A and B AB ii
None O

28. The ABO Blood Group
The gene for blood type, gene l, codes for a molecule that attaches to a membrane protein found on the surface of red blood cells.
The lA and lB alleles each code for a different molecule.
Your immune system recognizes the red blood cells as belonging to you. If cells with a different surface molecule enter your body, your immune system will attack them.

29. Surface molecule A is produced.
Phenotype A
Surface molecule A
The lA allele is dominant to i, so inheriting either the lAi alleles or the lA lA alleles from both parents will give you type A blood.
Surface molecule A is produced.

30. The lB allele is also dominant to i.
Phenotype B
Surface molecule B
The lB allele is also dominant to i.
To have type B blood, you must inherit the lB allele from one parent and either another lB allele or the i allele from the other.
Surface molecule B is produced.

31. The lA and lB alleles are codominant.
Phenotype AB
Surface molecule B
The lA and lB alleles are codominant.
This means that if you inherit the lA allele from one parent and the lB allele from the other, your red blood cells will produce both surface molecules and you will have type AB blood.
Surface molecule A

32. Phenotype O
The i allele is recessive and produces no surface molecules.
Therefore, if you are homozygous ii, your blood cells have no surface molecules and you have blood type O.

33. Sex Determination
In humans the diploid number of chromosomes is 46, or 23 pairs
There are 22 pairs of homologous chromosomes called autosomes. Homologous autosomes look alike.
The 23rd pair of chromosomes differs in males and females and are known as the sex chromosomes and are indicated by the letters with X and Y

34. If you are female, your 23rd pair of chromosomes are homologous, XX.
Sex Determination
If you are female, your 23rd pair of chromosomes are homologous, XX. X X
Female
If you are male, your 23rd pair of chromosomes XY, look different. X Y
Male

35. Sex Determination XY Male X Y X XX Female XY Male XX Female X

36. Sex Linked Inheritance
Traits controlled by genes located on sex chromosomes are called sex-linked traits
The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes
Because the X and Y chromosomes are not homologous, the Y chromosome has no corresponding allele to one on the X chromosome and no superscript is used

37. Dystrophy sex linked cross
White-eyed male (XrY) F2
Females:
Red-eyed female (XRXR)
all red eyed
Males:
1/2 red eyed
1/2 white eyed
F1 All red eyed

39. Sex-Linked Inheritance
The pattern of sex-linked inheritance is explained by the fact that males, who are XY, pass an X chromosome to each daughter and a Y chromosome to each son
Traits that are governed by X-linked recessive inheritance in humans are
1. red-green color blindness
2. Hemophilia
3. Muscular Dystrophy

40. Red-Green Color Blindness
People who have red-green color blindness can’t differentiate between these two colors.
This is caused by the inheritance of a recessive allele at either of two gene sites on the X chromosome.

41. Hemophilia
Hemophilia is an X-linked disorder that causes a problem with blood clotting.
About one male in every has hemophilia, but only about one in 100 million females inherits the same disorder
Males inherit the allele on the X chromosome from their carrier mothers. One recessive allele for hemophilia will cause the disorder in males
Females would need two recessive alleles to inherit hemophilia

43. Skin Color: polygenic inheritance
In the early 1900s, the idea that polygenic inheritance occurs in humans was first tested using data collected on skin color.
Scientists found that when light-skinned people mate with dark-skinned people, their offspring have intermediate skin colors

44. Skin color: A polygenic trait
This graph shows the expected distribution of human skin color if controlled by one, three, or four genes.
Number of Genes Involved in Skin Color
Expected distribution- 4 genes
Observed distribution of skin color
Expected distribution- 1 gene
Number of individuals
Expected distribution- 3 genes
Light
Right
Range of skin color

45. Polygenic Inheritance
Polygenic inheritance occurs when 1 trait is governed by 2 or more sets of alleles
A hybrid cross for skin color provides a range of intermediates
This also includes cleft lip, clubfoot, hypertension, diabetes, schizophrenia, allergies & cancers
Behavioral traits include suicide, phobias, alcoholism, and maybe homosexuality (not predetermined

46. Karyotype
This chart of chromosome pairs is called a karyotype, and it is valuable in identifying unusual chromosome numbers in cells

47. Some inheritable disorders that can be detected by using a Karyotype

48. 1. Downs Syndrome 21 or Trisomy 21
Down syndrome is the only autosomal trisomy in which affected individuals survive to adulthood
It occurs in about one in 700 live births
Down syndrome is a group of symptoms that results from trisomy of chromosome 21
Some degree of mental retardation

49. 2. Turners Syndrome
Turner (XO) syndrome females have only one sex chromosome, an X.
Turner females are short, have a broad chest and webbed neck.
Ovaries of Turner females never become functional; therefore, do not undergo puberty

50. 3. Klinefelter Syndrome
Klinefelter syndrome males have one Y chromosome and two or more X chromosomes.
Affected individuals are sterile males; testes and prostate are underdeveloped.
Individuals have large hands and feet and long arms and legs.

51. 4. Triplo-X Females Triplo-X females have three or more X chromosomes.
There is no increased femininity; most lack any physical abnormalities.
There is an increased risk of having triplo-X daughters or XXY sons.
May experience menstrual irregularities, including early onset of menopause.

52. 5. Jacob Syndrome
XYY males with Jacob syndrome have two Y chromosomes instead of one.
Results from nondisjunction during meiosis II.
Usually taller than average; suffer from persistent acne; tend to have lower intelligence.
Earlier claims that XYY individuals were likely to be aggressive are not correct