1. Human Genetics
Chapter 15: The Chromosomal Basis of Inheritance

2. Genes & Chromosomes
Mendel’s “hereditary factors” were , though this wasn’t known at the time
Today we can show that genes are
The location of a particular gene can be seen by tagging isolated chromosomes with

3. Chromosomal Theory of Inheritance
Mitosis and meiosis were first described in the
The chromosome theory of inheritance states:
The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

5. Experimental Evidence
The first solid evidence associating a specific gene with a specific chromosome came from
Morgan’s experiments with fruit flies provided convincing evidence that

6. Experimental Evidence
In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type or normal)
The F1 generation
The F2 generation showed the 3:1 red:white eye ratio, but
Morgan determined that the white-eyed mutant allele must be located
Morgan’s finding supported the

7. Sex linkage Sex chromosomes determine XX in females, XY in males
Each ovum contains an , while a sperm may contain
The SRY gene on the Y chromosome codes for the
X chromosome has genes for many traits

9. Sex-linked Inheritance
A gene located on either sex chromosome is called a
In humans, sex-linked usually refers to a gene on the
If gene is on
Sons will inherit from
Females don’t get Y-linked traits
Y-linked genes

10. X-linked Inheritance
If gene is on
Can inherit from
Sons always get

11. X-linked traits
AKA Bubble boy disease

12. X-linked recessive genes
Sex-linked genes follow specific patterns of inheritance
For a recessive sex-linked trait to be expressed
A female
A male
Sex-linked recessive disorders are much

13. Females & X-linked
In mammalian females, one of the two X chromosomes in each cell is
The inactive X condenses into a
If a female is heterozygous for a particular gene located on the X chromosome, she will be a

14. Carriers Females can be Have one copy of gene, Other X has
Males cannot be carriers, they either
Males will give gene to
If he has the gene all his daughters will be

15. Red-green color blindness
X-linked disorder
Can’t differentiate these two colors
Many people who have this are
First described in a boy who could not be trained to harvest only the ripe, red apples from his father’s orchard.
What serious consequence could result from this?

16. Sex-Linked Traits:
 1. Normal Color Vision:  A: 29,  B: 45,  C: --,  D: 26
 2. Red-Green Color-Blind:  A: 70,  B: --,  C: 5,  D: --
 3. Red Color-blind:  A: 70,  B: --,  C: 5,  D: 6
 4. Green Color-Blind:  A: 70,  B: --,  C: 5,  D: 2

17. Hemophilia An X-linked disorder that causes a
If your blood didn’t have the ability to clot and you bruised yourself or scraped your knee,
Queen Victoria was a carrier and she passed the trait on to some of her children

18. Hemophilia
About has hemophilia, but only about inherits the same disorder
Why????
Males only have
A single recessive allele for hemophilia will
Females would need to inherit hemophilia
Males inherit the allele for hemophilia on the X chromosome from their

19. Hemophilia

20. Hemophilia
Hemophilia can be treated with , the blood-clotting enzyme that is absent in people affected by the condition
Both treatments are
New methods of DNA technology are being used to develop a

21. Sex-linked Questions
Both the mother and the father of a male hemophiliac appear normal. From whom did the son inherit the allele for hemophilia? What are the genotypes of the mother, the father and the son?
A woman is color blind. If she marries a man with normal vision, what are the chances that her daughter will be color blind? Will be carriers? What are her chances that her sons will be color blind?
Is it possible for two normal parents to have a color blind daughter?

22. What is on our chromosomes?
Each chromosome has
Genes located on the same chromosome that tend to be inherited together are called
Thomas Morgan found that body color and wing size of fruit flies are usually inherited together in
He noted that these genes do not , and reasoned that they were
However, nonparental phenotypes were also produced
Understanding this result involves exploring

23. Genetic Recombination
Mendel observed that combinations of traits in some offspring differ from either parent
Offspring with a phenotype matching one of the parental phenotypes are called
Offspring with nonparental phenotypes (new combinations of traits) are called
Morgan discovered that genes can be linked, but the linkage was
Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome
Mechanism was the

24. Genetic map Alfred Sturtevant, one of Morgan’s students, constructed a
Sturtevant predicted that the farther apart two genes are, the and therefore the higher the recombination frequency

25. Genetic map A linkage map is a genetic map of a chromosome based on
Distances between genes can be expressed as ; one map unit, or centimorgan, represents a
Map units indicate , not precise locations of genes

26. Human Genome Project
The most ambitious mapping project to date has been the sequencing of the
Officially begun as the Human Genome Project in , the sequencing was largely completed by
The project had three stages:

27. Human Genome Project
A physical map expresses the distance between genetic markers,
It is constructed by cutting a DNA molecule into many short fragments and arranging them in order by identifying overlaps
Sequencing was then done on the chromosomes

28. Gene Manipulation
DNA sequencing has depended on advances in technology, starting with making recombinant DNA
In recombinant DNA, nucleotide sequences from
Methods for making recombinant DNA are central to genetic engineering,

29. Biotechnology DNA technology has revolutionized biotechnology,
One benefit of DNA technology is identification of human genes in which
Scientists can diagnose many human genetic disorders by using molecular biology techniques to
Genetic disorders can also be tested for using

30. Transgenics
Advances in DNA technology and genetic research are important to the development of
Transgenic animals are made by introducing genes from
Transgenic animals are pharmaceutical “ ,” producers of large amounts of otherwise rare substances for medical use
“Pharm” plants are also being developed to make human proteins for medical use
This is useful for the production of

31. Gene Therapy Gene therapy is the alteration of an
Gene therapy holds great potential for treating
Vectors are used for (example = bone marrow)
Gene therapy raises ethical questions, such as whether human germ-line cells should be treated to correct the defect in future generations

32. Causes of Genetic Disorders
Meiosis usually functions accurately, but problems may arise at times
Large-scale chromosomal alterations often lead to
In nondisjunction, pairs of homologous chromosomes
May occur in
One gamete receives
Another gamete receives

34. Fertilization after nondisjunction
Nondisjunction results in gametes with an extra or missing chromosome
If the other gamete is normal, the zygote will have
Most of the time an extra chromosome
results from the fertilization of gametes in which nondisjunction occurred
Offspring with this condition have an

35. Fertilization after nondisjunction
occurs when the zygote has only one copy of a particular chromosome (2n -1)
occurs when the zygote has three copies of a particular chromosome (2n+1)
is a condition in which an organism has more than two complete sets of chromosomes
is three sets of chromosomes
is four sets of chromosomes
Polyploidy is common in
Polyploids are in appearance than aneuploids

36. Nondisjunction animation

37. Human Disorders due to chromosome alterations
are associated with some serious disorders
Some types of aneuploidy appear to upset the genetic balance less than others, resulting in
These surviving individuals have a set of symptoms,

38. Down Syndrome
Down syndrome is an aneuploid condition that results from
Most common serious birth defect
Varying degrees of
Due to on 21st chromosome
1/2 eggs of female will carry extra 21 and 1/2 will be normal
Risk increases

40. Incidence of Down Syndrome

41. Klinefelter Syndrome Klinefelter syndrome is the result of an
Could be from nondisjunction

42. Turner Syndrome Turner syndrome produces
It is the only known viable monosomy in humans
Girls with Turner Syndrome do not develop

43. Mutation types may also lead to genetic disorders
Breakage of a chromosome can lead to four types of changes in chromosome structure:
Deletion
Duplication
Inversion
Translocation

44. Mutation types

45. Cri du chat
The syndrome cri du chat (“cry of the cat”), results from a
A child born with this syndrome is
Individuals usually die

46. Chronic Myelogenous Leukemia
Certain cancers, including chronic myelogenous leukemia (CML), are caused by
Occurs with the exchange of a large portion of with a small fragment from the tip of
Shortened, easily recognizable

47. Genomic imprinting
There are two normal exceptions to Mendelian genetics
One exception involves , and the other exception involves
Genes marked in gametes as coming from
Genes inherited from father than genes inherited from mother
For a small fraction of mammalian traits, the for those traits
Such variation in phenotype is called genomic imprinting
Example =

48. Organelle genes
Extranuclear genes (or cytoplasmic genes) are genes found
Mitochondria, chloroplasts, and other plant plastids carry
Extranuclear genes are inherited

49. Organelle genes
The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant
Some defects in mitochondrial genes prevent cells from making enough ATP and result in
For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy

50. Review Questions
State the 2 basic ideas behind the chromosomal theory of inheritance.
Explain Morgan’s experiment and how it gave evidence that genes are located on chromosomes.
Explain sex linkage and sex-linked inheritance.
Name and describe characteristics of 4 genetic diseases that are known to be X-linked.
Explain the idea of a “carrier” for an X-linked genetic disease.
Carry out a monohybrid cross of an X-linked trait using a Punnett square.
Explain the idea of linked genes.
Explain the result of genetic recombination.
Identify the significance of genetic maps and linkage maps
Describe the Human Genome Project and differentiate between its 3 main stages.
Discuss the advantages of gene manipulation and biotechnology.
Describe various uses of transgenic animals.
Explain the purpose and use of gene therapy.
Explain how errors in meiosis can cause genetic syndromes.

51. Review Questions Define nondisjunction.
Differentiate between aneuploidy, monosomy, trisomy, and polyploidy.
Explain the cause, frequency, and problems associated with the following genetic syndromes: Down syndrome, Klinefelter syndrome, & Turner syndrome.
Describe the effect of mutations on genes.
Differentiate between deletion, duplication, inversion, and translocation mutations.
Explain cri du chat syndrome.
Explain chronic myelogenous leukemia as an example of a disease-causing mutation.
Explain genomic imprinting and the effects of extranuclear genes.