Human Chromosomes & Genetics

I. Intro to Human Genetics

A. Of all the living things, there is one in particular that has always drawn our interest, that creature is Homo sapiens.

B. Scientists once knew much less about humans than about other organisms.

C. Scientists are now on the verge of understanding human genetics at least as well as they understand that of some other organisms.

II. Human TraitsA. Human genes are inherited

according to the same principles that Mendel discovered.

B. In order to apply Mendelian genetics scientists have to figure if the trait is controlled by single gene

C. Human traits however are usually polygenic, controlled by many genes

D. Many traits are influenced by environmental, or nongenetic factors, i.e. nutrition & exercise

III. Human Chromosomes

A. Biologists can analyze chromosomes and identify what organism they are studying just by simply counting and organizing the chromosomes in pairs.

B. A picture of chromosomes arranged this way is called a karyotype

C. Human karyotypes have 23 pairs of chromosomes each coming from 2 haploid cells1. 22 pairs of autosomal chromosomes and 1 pair of sex

chromosomes

Karyotype

III. Human Chromosomes

D. Determining sex of an offspring (50:50)1. All human egg cells carry a single X chromosome

(23,X)2. Half of all sperm cells carry an X chromosome

(23,X) and half carry a Y chromosome (23,Y).

IV. Sex-Linked Genes• A. Sex-Linked genes are genes located on the X or Y

chromosome. • B. Many genes are found on the X chromosomes (over

100 sex-linked genetic disorders are mapped to the X chromosome)

• C. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive

• D. Examples:– 1. Colorblindness (found in about 1 in 10 males)– 2. Hemophilia (blood does not clot properly)– 3. Duchenne Muscular Dystrophy

• Females can be-carriershave itnot have it

• Males either-have it or not

V. PedigreeA. A pedigree is a chart, which shows the

relationships within a family, can be used to help with how traits get passed from one generation to the next. Looking at phenotypes the mode of inheritance can be determined.

VI. Human Genes & Chromosomes• A. Of the 46 chromosomes, chromosomes 21 & 22 are

the smallest autosomal chromosomes and were there first chromosomes to be completely sequenced.– 1. Chromosome 22 contains as many as 545 different genes,

some are very important for health. Genetic disorder found on 22 is a form of leukemia & neurofibromatosis

– 2. Chromosome 21 contains 225 genes and can carry the genetic disorder known as ALS

• B. Molecular biologists gradually learn more about how the arrangements of genes on chromosomes affect gene expression and development

VII. X-Chromosome Inactivation• A. Cells only need one X-chromosome to function

properly in males, in females to adjust to having 2 X-chromosomes 1 X-chromosomes is randomly switched off. (Each somatic cell during development)

• B. Barr bodies are dense regions in the nucleus of an inactive X-chromosome.

• C. Mammal’s hair color (cats) is carried on the X-chromosome– 1. 2 different cells can have 2 different X-chromosomes

turned on

IIX. Epistasis• A. Two separate genes control one trait, but one

gene masks the expression of the other gene. The gene that does the masking is epistatic. – 1. 2 genes control different enzymes in 2 different

pathways. Example gene A and gene C. – C is epistatic to A

IX. Human Genome: Autosomal DisordersThe human genome (complete set of genetic info)

includes tens of thousands of genesA.First genes discovered were the genes for blood

type1. ABO blood groups2. Rh gene

IX. Human Genome : Autosomal DisordersB. Autosomal Recessive Alleles – genetic disorder that

are caused when an abnormal or nonfunctioning alleles affect the phenotype

Examples:PKU-(phenylketonuria) can not break down this amino acid.

Carried on chromosome 12 (elimination from diet )Tay-Sachs – breakdown of central nervous system and death in

the first few years. Can not break down lipids needed for brain function.

Cystic Fibrosis – caused by a recessive gene on chromosome 7 (also follows pleiotropy – affect organisms in more than 1 way)

IX. Human Genome : Autosomal Disorders

C. Autosomal Dominant Alleles – these traits will be present if just only one allele is inherited

ExampleAchondroplasai - a form of dwarfism Huntington’s disease - nervous system disorder, that

usually shows its symptoms in an individuals late thirties

IX. Human Genome : Autosomal DisordersD. Codominant Alleles - allelic products co-exist in the

phenotypeExamples

Sickle cell Anemia (recessive)- mutation in hemoglobin someone with both normal and sickle cell anemia has a different phenotype (resistant to malaria)

X. Chromosomal Disorders• A. During meiosis when a chromosome fails to

separate is known as nondisjunction.– 1. If this occurs abnormal number of chromosomes will be in

an individuals gametes (during fertilization the zygote is aneuploidy )

– 2. Can be caused by: deletion, inversion, translocation or polyploidy

• B. Examples– 1. Down Syndrome – 2 copies of the autosomal

chromosome do not separate, giving an individual 3 copies of the 21st chromosome known as trisomy

XI. Sex Chromosome Disorders

• A. If nondisjunction occurs to a gametes sex chromosomes

• B. Examples include:– 1. Turner’s syndrome – a female inherits only 1 X

chromosome (45,X) – 2. Klinefelter’s syndrome – a male inherits an

extra X (47, XXY)– 3. XYY Syndrome – male inherits an extra Y

chromosome

Turner’s Syndrome

Klinefelter’s syndrome