Ch 14

Human Heredity

14.1

Human Chromosomes

Karyotypes

A Human Karyotype
A typical human cell has 23 pairs of chromosomes. These chromosomes have been cut out of a photograph and arranged to form a karyotype.

A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size. Scientists the but out the chromosomes from the photographs and arrange them in a picture.


Genome - The full set of genetic information that an organism carries in its DNA.

Sex Chromosomes - Two of the 46 chromosomes in the human genome are known as sex chromosomes, because they determine the sex of the individual. 

Males have one X chromosome and one Y chromosome (XY)

Females have two X chromosomes (XX)

Males and females are born in a roughly 50:50 ratio.

All human egg cells carry a single X chromosome.

Half of all sperm cells carry an X chromosome (23,X)
and half of all sperm cells carry a Y chromosome (23, Y)

This ensures that just about half the zygotes will be males and half will be females.

More than 1200 genes are found on the X chromosome. Not all of these are gender related.
The Y chromosome is much smaller  than the X chromosome and contains only about 140 genes, most of which are associated with male sex determination and sperm development.


Autosomes
The remaining 44 human chromosomes (pair 1-22) (not sex chromosomes) are called autosomes.

So, a human genome consists of 44 autosomes and 2 sex chromosomes.

Human Chromosomes
Human chromosomes follow several patterns of inheritance including:
Dominant and Recessive Alleles
Codominant Alleles
Incomplete Dominant Alleles
Multiple Alleles

Sex-Linked Inheritance
Genes that are located on the X and Y chromosomes are considered "sex-linked", even if they have nothing to do with the determination of sex.

Genes found on the Y chromosome are passed directly from father to son.

Because males only have one X chromosome, if there is a defect in a gene located on it, then the defective gene will be expressed. But, since females have two X chromosomes, a defective gene on only one of those chromosomes won't be expressed if it is recessive. Therefore, females can "carry" traits that may only show up in their sons if the son receives the defective chromosome. 

Colorblindness is one of those sex-linked chromosomes that usually only shows up in men who have received the defective X chromosome from their mother. If a female receives two defective alleles for a recessive disease, (one from each parent) then the female would express the disease. 
Because of this, colorblindness is much more prevalent in males (1 in 12) than in females (1 in 200).

X Chromosome Inactivation
Since females have two X chromosomes, female cells adjust by randomly switching off one of them. The "switched off" chromosome condenses and forms a "Barr Body" in the nucleus. Barr bodies are only found in females.
An example of how two different alleles (one on each X chromosome) show up in different locations on the body of the same organism is that of a calico cat. This cat has two different colored spots. The gene for spots is found on the X chromosome. In some locations on the cat's body, the X chromosome with the orange spot allele will be shut off, whereas, in other locations, the other X chromosome carrying the black spot allele would be shut off. The cat would have orange spots where the black allele is shut down, and black spots where the orange allele is shut down.

Pedigrees
A pedigree is a chart that shows the relationship within a family.  A pedigree shows the presence or absence of a trait according to the relationships between parents, siblings, and offspring.

The information gained from pedigree analysis makes it possible to determine the nature of genes and alleles associated with inherited human traits. Based on a pedigree, you can often determine if an allele for a trait is dominant or recessive, autosomal or sex-linked.


14.2
Human Genetic Disorders
Genes are expressed in the proteins they make.
Changes in a gene's DNA sequence can change proteins by altering their amino acid sequences, which may directly affect one's phenotype. 

Sickle Cell Disease
Sickle Cell Anemia is a disorder caused by a defective allele for a protein that makes up hemoglobin (Hg). The defective protein makes the Hg a bit less soluble, causing the Hg molecules to stick together when the blood's oxygen level decreases. The molecules club into long fibers, forcing cells into a distinctive sickle shape, which gives the disorder it's name.
Sickle shaped cells are more rigid than normal red blood cells and they tend to get stuck in the capillaries. If the blood stops moving through the capillaries, damage to cells, tissues, and organs can result.

Sickle cell is a recessive gene prevalent in African descendants. It shows up early in life.

Interestingly enough, people who have one or two sickle cell alleles are immune to another disease found primarily in Africa, Malaria.

Cystic Fibrosis
Another recessive disease, Cystic Fibrosis, is most common among European ancestry. CF is caused by the deletion of just three nitrogen bases in the gene for a protein which is involved in Cl- ion transport across the cell membrane. Since water follows salt, the lack of water causes the mucous membranes to secrete a very thick mucous which causes many problems including clogged ducts, gas exchange, and pneumonia.

People with one normal copy of the CF allele are unaffected by CF, because they can produce enough CFTR to allow their cells to work properly. Two copies of the defective allele are needed to produce the disorder because it is recessive. CF shows up early in life. The life expectancy is shortened.

People who carry the CF gene are resistant to Typhoid Fever caused by a bacteria.

Huntington's Disease
Huntington's is caused by a dominant allele for a protein found in brain cells. The allele for this disease contains a long string of bases in which the sequence CAG repeats itself over and over again. The more it repeats itself, the more severe the disease and the earlier the onset. However, this disease does not normally show itself until middle age. Huntington's Disease results in mental deterioration and uncontrollable movements.

Chromosomal Disorders

Most of the time, meiosis works perfectly and each human gamete gets exactly 23 chromosomes. But, every so often, an error in the separation of the chromosomes occurs. Nondisjunction which means "not coming apart" is the most common error in meiosis and occurs when the homologous chromosomes don't separate properly.
If nondisjunction occurs during meiosis, gametes with an abnormal number of chromosomes may result, leading to a disorder of chromosome numbers. 
If a gamete ends up with 3 homologous chromosomes, this is called a trisomy (three bodies).  The most common form of trisomy, involving 3 copies of chromosome 21, is Down syndrome.

Turner's Syndrome
A female with Turner's Syndrome usually inherits only one X chromosome. Women with Turner's syndrome are sterile and their sex organs do not develop properly at puberty.

Klinefelter's Syndrome

Males with this syndrome inherit an extra X chromosome which interferes with meiosis and usually prevents these individuals from reproducing. 

Down Syndrome (Trisomy 21)

***There have been no reported instances of babies being born without and X chromosome, indicating that this chromosome contains genes that are vital for the survival and development of the embryo.

For 14.1 powerpoint click here:

https://docs.google.com/presentation/d/1JzbGxyJmz7mtiQYWAVrr_vS56D7fCFrJxAbPUjSno70/edit?usp=sharing

For 14.2 powerpoint click here:

https://docs.google.com/presentation/d/17GavtFFnRE26oYpB9fMGxaABy5MNRd_-M3PdV-LHF6A/edit?usp=sharing