HUMAN GENETICS

Karyotypes and Pedigrees

Inheritance of Traits

Karyotypes

Definitions• Karyotype – a picture of

an individual’s chromosomes.• Karyotyping - A

technique that uses a picture of an individual’s chromosomes to analyze and detect chromosomal abnormalities.

Uses1. To detect possible fetal

abnormalities.

2. Form of genetic screening conducted on potential parents.

Methods for Preparing a KaryotypeFetus

Pre-natal diagnosis1. Amniocentesis- a small

amount of amniotic fluid, which contains fetal tissues, is sampled from the amniotic sac surrounding a developing fetus, and the fetal DNA is examined for genetic abnormalities.

(see diagram)

2. Chorionic villi sampling- entails sampling of the placental tissue and testing it for chromosomal abnormalities.

Individuals• Postnatal diagnosis• White blood cell test

Making the Karyotype

Arrangement of a Karyotype• Humans have 23 pairs (46) of chromosomes

• 22 pairs of autosomes (body cells)

• 1 pair of sex chromosomes – which determine the sex of the individual• Females are XX; males are XY

• Arranged by size from largest to smallest• 1 pair of homologous chromosomes at each site

Types of Chromosome Abnormalities

1. Nondisjunction – failure of homologous pairs of chromosomes to separate during meiosis.• Results in trisomy – 3 chromosomes instead of a pair at a given

site.2. Deletion – missing segment of a chromosome.

monosomy – missing a whole chromosome3. Translocation – segment of one chromosome is attached to

another chromosome.• Crossing over – exchange of parts between two homologous

chromosomes.4. Inversion – genes switch loci (places) on a chromosome.

Syndrome – a group of symptoms.

Normal Male(46,XY)

Normal Female (46,XX)

Down’s Syndrome

Edward’s Syndrome

Patau’s Syndrome

Turner Syndrome (45,XO)

Cri du chat Syndrome (partial deletion of chromosome

5)

Try this one

15:21 Translocation

Incidence of Chromosomal Abnormalities by Mother’s Age Mother’s Age Incidence per 1000 births

20 1.9

25 2.1

30 2.5

35 4.9

40 13.7

45+ 43.4

Incidence of Trisomy 21 by Mother’s Age

Age Incidence per 1000 births

20 0.6

25 0.8

30 1.1

35 2.7

40 9.2

45+ 30.8

Classification of Genetic Disorders

• Single Gene Defect• Autosomal dominant• Autosomal recessive• X-linked dominant• X-linked recessive

• Chromosomal Abnormalities• Number• Structure

• Multifactorial Defects

Pedigree Charts

Carriers – individuals that have the gene for a trait but do not have the condition; heterozygous

Patterns of Inheritance1. Autosomal Dominant• Does not skip generations• Affects males and females equally• Approximately ½ the offspring of a person with the trait will be

affected• The affected person is usually heterozygous

2. Autosomal Recessive• Both parents and grandparents of an affected person are usually

normal.• ¼ of the siblings of an affected person will also be affected.• Affects males and females equally.

Patterns of Inheritance3. X-linked Dominant• Affects females more than males• Affected males have no normal daughters and no affected sons• Affected females who are homozygous transmit the trait to all

children• Does not skip generations

4. X-linked Recessive• Skips generations• Affects males more than females• Trait is transmitted by “carrier” females• An affected male will pass the carrier status to all of his daughters

so ½ of their sons will be affected.

X-linked Inheritance• Genes on the X chromosome have no counterpart on the Y chromosome,

therefore, characteristics determined by a gene on the X chromosome are always expressed in males.

How many males are affected?How many females are affected?What is the pattern of inheritance?

Pattern of inheritance?