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Topics for January 21

Results of class assessment #2.

Genetics and humans: Mendelian principles apply.

Tools: karyotype and pedigree analysis.

Chromosomes and linkage groups.

Autosomes and sex chromosomes.

Genetic abnormalities: gene disorders or chromosome aneuploidies.

Results of CAT #2

Human Genetics: Mendelian principles apply.

Mendelian principles apply to humans.

Patterns include:

Chromosomes are the location of genes.

Genetically determined characteristics display family history patterns.

Some human maladies are the result of particular gene defects or chromosome disorders.

Some examples of simple Mendelian genetic traits.

Attached (recessive) vs. free (dominant) ears. (p 138).

Tongue-rolling (dominant) vs. non-tongue-rolling (recessive). (p154).

Achoo syndrome (dominant) vs. not (recessive). p154.

Albinisim (recessive) vs. not (dominant). p155.

Human health and human genetics.

Genetic abnormality: a rare or uncommon version of a trait. E.g. extra toes or fingers.

Genetic disorder: an inherited condition that results in mild to severe medical problems.

"Diseases" result from infectious agents such as bacteria or viruses. Genetic traits are heritable but not communicable.

Tools applied to human genetics.

Experiments are not possible!

1. Karyotype analysis:

The karyotype is a "family a persons chromosomes.

2. Pedigree analysis:

Genetic traits, including genetically determined maladies, follow family patterns which reveal the nature of the genetic mechanism.

The karyotype (see 10.2, p159)

The procedure begins with a sample of some intact cells: blood, skin, bone, etc.

The sample is cultured in vitro (to obtain a large number of cells).

Dividing cells (in the culture) are treated with a chemical (colchicine) which stops cells at metaphase (of mitosis).

Karyotype (cont.)

Cell culture is centrifuged (to concentrate the cells).

Cells are transferred to a hypotonic fluid (low salt solution) which makes them swell.

Cells are placed on a microscope slide, dried, and stained (to reveal the chromosomes).

Karyotype (cont.)

A cell that clearly shows all the chromosomes is photographed (through the microscope).

The photograph is developed and printed.

The image of each chromosome is cut out of the photo, and all the images are arranged by homologous pairs, biggest first, smallest last.

Karyotype (cont.)

The final product: All the chromosomes, by homologous pairs, from largest to smallest. (With the clever names of chromosome #1, chromosome #2, etc.)

Exception: one unique homologous pair determines gender. Hence, this pair is called the sex chromosomes, X and Y.

Karyotype: the final product.

The karyotype is a portrait of a person’s chromosomes.

The commonest pattern is: 22 pairs of autosomes (that don’t directly determine gender) and 1 pair of sex chromosomes.

Sometimes, a chromosome is missing or present as an extra copy.

Other patterns may be visible.

Uses of a karyotype.

A karyotype may detect certain genetic disorders. Example, an extra chromosome #21 (i.e. 3 copies of #21, not 2) will produce a pattern known as Down’s syndrome. Pre-natal samples are possible.

The banding pattern of individual chromosomes can reveal paternity or other family relationships. (other methods exist.)

Pedigree analysis: detecting genetic maladies.

Pedigrees of the family history of a genetic disorder often help identify the nature of the genetic cause (i.e. dominant or recessive gene, chromosome abnormality, etc.)

Famous example: Queen Victoria carried an X-linked recessive allele that causes hemophilia. Some of descendants suffered from the disorder (see p167).

Constructing a pedigree. (p164)

A pedigree is a diagram of the genetic characteristics of several generations of a family.

Specific symbols are used to indicate the pattern of relationship and the pattern of the appearance of the genetic trait.

Example: males are represented as small squares, females as small circles.

Utility of a pedigree.

A pedigree analysis will help reveal the nature of a genetic disorder. For example:

Dominant alleles show a different pattern than recessive alleles.

Genes located on the X chromosome reveal different patterns than genes located on autosomes.

Sex chromosomes and gender.

The pair of homologous chromosomes called the sex chromosomes are unique in two respects:

1. They are not equal in size. One is very much larger. Hence, they are given different names: X (large), Y(small)

2. They determine gender:

XX=female; XY=Male

X and Y chromosomes and their genes.

The X chromosome is a relatively large chromosome (about the size of #6).

Many genes not in any way related to sex or gender are located on the X chromosome. Example: genes for color vision, blood clotting chemicals, muscle function, etc.

The Y chromosome is very small.

The Y chromosome has few genes, and some of those are crucial for gender determination.

Examples of gene loci on the X chromosome of Drosophila.

white eye

crossveinless wings

miniature wings

forked bristles

fused veins

tan body

bobbed bristles

Examples of gene loci on the autosomes of Drosophila

Traits and gene loci

In general, there is no particular relationship between the trait affected by a particular set of alleles and the chromosome on which they are located.

Most genes located on the sex chromosomes have no relation to gender traits.

Exception: SRY gene on mammal Y chromosome determines male gender.

Sex chromosomes:
males and females.

Females have two copies of X. Hence, they are diploid for all genes located on the X. Normal patterns of dominant or recessive apply.

Males have just one X. Hence, they are "hemizygous" for the X chromosome. Recessive or dominant doesn’t apply, since males have just one X.

Gene located on a sex chromosome: eye color of Drosophila. (see p162)

Normal eye color in Drosophila is red, but some flies have white eyes.

The locus of the gene causing white eyes is on the X chromosome.

The pattern of inheritance of white eye is related to gender.

The trait is said to be "X-linked".

Drosophila eye color (cont.)

The allele for white eye color is recessive.

A female may have one of 3 different genotypes:

RR, and red-eye phenotype

Rr, and red-eye phenotype

rr, and white-eye phenotype

A male may have only one of two genotypes:

R, and red-eye phenotype

r, and white-eye phenotype

Patterns of inheritance eye color

Example in text:

Cross homozygous dominant female (RR) with recessive male (rY). (The Y has no allele for eye color.)

F1 flies are all red-eyed. Females are heterozygous, males are RY.

F2 flies are 1/4 white eyed males (rY). All the others have red eyes. (RR, or Rr females, or RY males.)

Understanding the patterns of X-linked genetic traits.

Genes with their locus on the X chromosome are said to be X-linked.

In a pedigree analysis of an X linked genetic abnormality (e.g. European royalty, p167), males are more likely to express an X-linked trait. Females may be "carriers".

Patterns of inheritance are a consequence of X linkage, dominant/recessive, etc.

X and Y chromosomes and traits related to gender.

Many genes located on the X chromosome have no relationship to gender related traits.

An important exception: One of the few genes located on the Y chromosome is the "SRY"gene. (For Sex-determining Region of the Y chromosome.) This gene is not on the X chromosome.

Result: XY causes male phenotype to develop. (see figure 10.4)

Y chromosome and development

The SRY gene, present on the Y chromosome, apparently functions as a master regulatory protein. It sets a cascade of reactions in motion that determine male development.

Many other genes, present on other chromosomes are part of the development of male and female reproductive characteristics.

Genetic disorders in humans.

Some genetic disorders are a consequence of the presence of particular alleles present at particular gene loci. (Gene disorder.)

Some genetic disorders are a consequence of consequence of wrong chromosome number. (Chromosome disorder.)

Gene disorders (see list p165)

Many examples of gene disorders and genetic abnormalities have been discovered.

The pattern of expression in a family pedigree is related to:

locus of the gene (autosome or X)

dominant or recessive

Possible patterns: Autosomal dominant or recessive, X-linked dominant or recessive.

Chromosome disorders.

Chromosome disorders may be a consequence of too many or too few chromosomes

Autosome disorders occur. Commonest type: Down’s syndrome.

Sex chromosome disorders occur. Common sex chromosome disorders are Turner syndrome and Kleinfelter syndrome.

Summary

Karyotype and pedigree analysis are useful for studying human genetics.

Gender is determined by the complement of X and Y chromosomes present.

Some genes are located on the X chromosome (X-linked).

Some genes are located on the autosomes.

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