1. Human Genetics: Patterns of Inheritance for Human Traits

2. Terms to know
Gene – segment of DNA. Carries genetic information that codes for a trait.
Chromosome - compact packages of DNA.
Inside nucleus.
Sperm – male sex cell (gamete)
Egg – female sex cell (gamete)

3. Terms to know Karyotype – arrangement of chromosomes. A picture.
Autosome - any chromosome that is not a sex chromosome

4. Terms to know:
Genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype.
A capital letter represents the dominant form of a gene (allele)
A lowercase letter is the abbreviation for the recessive form of the gene (allele).
Example:
V = dominant
v = recessive

5. Terms to know
Dominant: a gene that is expressed, regardless of whether its counterpart allele on the other chromosome is dominant or recessive.
Ex: VV or Vv = Long wings
Recessive: gene that produces its characteristic phenotype only when its allele is identical; vv= short wings

7. Terms to know:
Phenotype = the physical appearance of a trait in an organism
BB, Bb = brown eyes
bb = blue eyes

8. Terms to know
Allele: different versions of the same gene occupying a given position on a chromosome
Chromosome from Mom
Chromosome from Dad

9. Terms to know
Organisms that have 2 identical alleles for a particular trait are called homozygous (TT or tt).
Organisms that have 2 different alleles for a particular trait are called heterozygous (Tt).

10. What makes us human?
The use of a microscope to analyze cells during mitosis shows condensed chromosomes
Scientists cut pictures of these chromosomes out in and arrange them into a karyotype (right).
=The number and visual appearance of the chromosomes in the cell nuclei of an organism or species

11. The beginning
2 haploid gametes (egg and sperm) join together during fertilization.
The result is a fertilized egg called a zygote which is diploid.
Humans have 46 chromosomes

13. Male or Female?
Two of the 46 chromosomes are sex chromosomes because they determine whether the zygote will develop into a male or female
Females have XX chromosomes
Males have XY chromosomes

14. What about the other chromosomes?
The 44 chromosomes that are not sex chromosomes are called autosomes.

15. Which parent determines the sex of the baby?
Will it be a boy or a girl?
Because of the way sex chromosomes segregate, there is a 50:50 chance of both males and females being born
All human egg cells carry a single X chromosome, but only half of sperm cells carry an X while the other half carries a Y
Which parent determines the sex of the baby?

16. Human Traits
To identify an inherited trait controlled by a single gene, first we need to determine if that the trait is inherited and not a result of environmental influences. Then, we have to study how it’s passed from one generation to the next

17. Pedigrees
A pedigree chart shows the relationships within a family and how particular traits are passed from one generation to the next

18. So can you determine who gave you those good looks?
Nope. Unfortunately, many traits are polygenic, meaning they are controlled by many genes.
Example: eye or ear shape
Additionally many traits are influenced by the environment
Example: genetics determines a person’s maximum possible height, nutrition also plays a role in whether or not that height is ever reached

19. Important Note:
While it’s important to consider environmental effects on expression of genes…
Environmental effects on genes expression ARE NOT INHERITED!!!
Analogy: Just because someone’s arm gets cut off doesn’t mean their child will be born missing an arm!

20. YOUR TURN! Determine whether albinism is a dominant or recessive trait:aa Aa aa aa Aa Aa Aa Aa Aa
Recessive

21. Dominance
Dominant alleles will be expressed even if the recessive allele is present
Codominance: both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other
R = Red flower
W = White flower
RW = Red & White spotted flowers

22. Incomplete Dominance
Incomplete Dominance: both alleles of a gene are expressed in the phenotype- resulting in combined phenotype
red flowers crossed with white flowers
= pink flowers

23. How can you tell the difference?
The trick is to recognize when you are dealing with a question involving incomplete dominance.
There are two steps to this: 1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents. 2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

24. Chromosomal Disorders
When meiosis goes wrong and homologous chromosomes fail to separate its called nondisjunction This means “not coming apart.”
When this happens abnormal numbers of chromosomes end up in gametes and a disorder of chromosome numbers may result.
Examples: Down Syndrome, Turner’s Syndrome, Klinefelter’s Syndrome

25. Down Syndrome
If 2 copies of an autosomal chromosome fail to separate in meiosis an individual may be born with 3 copies of a chromosome. This is known as “trisomy”
Down Syndrome is the most common form of this, called Trisomy 21.

27. Child with Downs Syndrome
The extra chromosome causes problems with the way the body and brain develop.

28. Sex Chromosome Disorders
In females, nondisjunction can lead to Turner’s Syndrome
She inherits only 1 X (45, X)
Sterile and sex organs don’t develop at puberty
In males, nondisjunction can lead to Klinefelter’s Syndrome (47, XXY)
Extra X interferes with meiosis and usually prevents individuals from reproducing
Estimated 1 in every males
XXXY and XXXXY have been found!

29. So what happens if a gene sucks?
The Human Genome Project may help in finding cures for genetic disorders through gene therapy.
Gene therapy is the process of changing a gene that causes a genetic disorder. An absent or faulty gene is replaced by a normal, working gene.

30. Gene Therapy cont….
The most common form of gene therapy involves using DNA that encodes a functional, therapeutic gene in order to replace a mutated gene.
Today, most gene therapy studies are aimed at cancer and hereditary diseases linked to a genetic defect

32. Ethically Speaking
If scientists can find ways to cure genetic diseases is it okay if they also start engineering taller people, or their hair color, skin color, sex, blood group or appearance??????
What are the kinds of ramifications (effects) gene therapy in this way could have on human populations in the world?

33. What about lacking an X?
There have been NO incidences in which an individual lacks an X chromosome all together
This suggests that the X chromosome contains genes that are vital for the survival and development of an embryo

34. 5 Patterns of Inheritance for Human Traits
1. Single Allele Dominant
2. Single Allele Recessive
3. Sex Linked (X-Linked)
4. Multiple Alleles
5. Polygenic Traits

35. Single Allele Genes
Regular traits that are either determined by a dominant or recessive allele on an autosome
Autosomal Dominant examples:
a. Huntington’s Disease
b. Achondroplasia (dwarfisim)
c. Polydactyly (extra fingers and toes)
2. Autosomal Recessive examples:
a. Albinism
b. Cystic Fibrosis
c. Sickle Cell Anemia

36. 1. Dominant Allele Disorders
Huntington’s Disease (HD)
Results in loss of muscle control and mental deterioration
No signs are shown until 30’s
Brain degeneration
Treatment: No cure, but drug treatments are available to help manage symptoms.

37. 1. Dominant Allele Disorders
Achondroplasia
Dwarfism
Person grows no taller than 4’4

38. 1. Dominant Allele Disorders
Polydactyly
The presence of more than the normal number of fingers or toes.
Can usually be corrected by surgery.

39. 2. Recessive Allele Disorders
Albinism
Lack of pigment in skin, hair, and eyes
Mutation in one of several genes which provide the instructions for producing one of several proteins in charge of making melanin.

40. 2. Recessive Allele Disorders
Cystic Fibrosis (CF)
Caused by recessive allele on chromosome 7
Small genetic change (removes one Amino Acid)  changes protein
Results in: Excess mucus in the lungs, liver and digestive tract, gets infection easily, and early death unless treated.

41. 2. Recessive Allele Disorders
Sickle Cell Disease
Red blood cells are bent and twisted
Get stuck in capillaries  damage tissues
Results in weakness, damage to brain and heart

42. 3. Sex Link Traits: Genes on the X and Y chromosomes

43. how we determine gender…
How do we determine the sex of an individual?
 through sex chromosomes
Sex Chromosomes: X and Y
Female: XX
Male: XY
Who is responsible for gender determination in the child?
The father because he gives an X or Y to the gametes. The mother only gives an X to the gamete
Which sex chromosome is bigger?
 the X chromosome (y is much smaller)

44. What are sex-linked genes?  genes found on a sex chromosome
X-linked genes are genes found on the X chromosome, symbolized by Xr, XR, Y0.
Y-linked genes are found on the Y chromosome, symbolized by X0, YR, Yr
Thomas Morgan experimented with the eye color of fruit flies (Drosophilia) to determine X- linkage

45. Examples of X-linked traits:
1. Color Blindness 2. Muscular Dystrophy 3. Hemophilia 4. Icthyosis simplex (scaly skin)

46. Sex-Linked Genes
Turns out, there’s a special pattern of inheritance for genes located on the X or Y chromosome.
Because these chromosomes determine sex, genes located on them are termed sex-linked

47. Colorblindness
About 1 in 10 males is color blind, while 1 in 100 females are color blind in the U.S.
Colorblindness is X-linked. This means that the genes for color vision are located on the X chromosome.
Since females have 2 Xs they have a better shot at color vision, while males only have 1 X which means all X-linked alleles are expressed even if they are recessive.

48. Colorblindness

49. Colorblindness
A person with normal color vision sees a number seven in the circle above.
Those who are color blind usually do not see any number at all.

50. Colorblindness RED-GREEN COLORBLINDNESS:
People with red-green color blindness see either a three or nothing at all.
Those with normal color vision see an 8.

51. Duchenne Muscular Dystrophy
Sex-linked
Results in progressive weakening and loss of skeletal muscle
1 in 3,000 U.S. males is born with it

52. What happens to the extra X in females?
Males only get one copy of the X chromosome, so what happens to the extra one in females?
One X is randomly turned “off”. This is called X-Chromosome Inactivation. It forms a dense region in the nucleus called a Barr Body

53. This happens in cats!
One X chromosome may have the allele for orange spots while the other X may have the allele for black spots
In cells in different parts of the body one X may be turned on while the other X is turned on elsewhere
The cat’s fur will have a mixture of orange and black spots
Fact: If a cat has 3 colors, it’s female. Males can only have 2 colors (unless they have genetic disorder)!

54. Hemophilia Hemophilia- Lacking in the ability to clot blood
There is a gene on the “X” chromosome that controls blood clotting
People who have hemophilia are missing the protein to clot blood
They can bleed to death by a minor cut.

55. Muscular Dystrophy - Results in weakening/loss of muscles
- Caused by defective version of gene that codes for muscle

56. Sex-influenced Traits
Sex-Influenced traits are those that are on autosomes, but occur because of the sex hormones in male and female bodies.
Examples: Facial hair
Baldness

57. 4. Multiple Alleles
Multiple Alleles – any gene that has 3 or more alleles (not just 1 dominant and 1 recessive)
Example: Blood type has 3 alleles:
IA= Type A blood (dominant)
IB= Type B blood (dominant)
i = Type O blood (recessive)

58. Check out the possible genotypes and phenotypes of blood below:
IAIA or IAi
Type A
IBIB or IBi
Type B
IAIB
Type AB ii
Type O

59. Blood has both Multiple Alleles and is CoDominant
If you have IAIB as your genes, you have both Type A and Type B blood, also known as Type AB
If you have IAi, i is recessive to IA, so you have type A blood
Q. When would you have Type O blood?
A. When you have ii as your genotype.

60. 5. Polygenic Traits – traits controlled by 2 or more genes that interact, forming the trait
Usually show a wide range of phenotypes
Ex: Skin color, eye color, foot size, height
Wide range of skin colors because there are more than 4 genes that control this trait.
These may also be influenced by the environment, for example height. If not given the proper nutrition as a child, they might not be as tall as their genes dictate.