Chapter 24: Chromosomal Basis of Inheritance

23

Amount of pairs of chromosomes male and female humans normally have

Autosomes

Not sex chromosomes, 22 pairs

Sex chromosomes

one pair

XY

human male chromosomes

XX

human female chromosomes

Nonhomologous

X and Y chromosomes which contain different combinations of genes

Sex-linked traits

Controlled by genes on the sex chromosomes

X-linked traits

Found on the X chromosomes, 2,000 genes but do not not play a role in female reproduction, no matching gene on the Y

Y-linked genes

Found on the Y chromosomes, 200 genes, many genes help to determine sex, have a sex determining region called SRY gene

X-linked

Most of their genetic disorders are recessive

Male

Always receives the X-linked allele from his mother from her X chromosome in x-linked traits

Y chromosome

This chromosome from the father does not carry an allele for the trait

two recessive alleles

Must be received by females for genetic disorders, one from each parent

X chromosome

In sex-linked alleles when examining x-linked traits the allele on this chromosome is shown as a letter attached to the X chromosome

Carrier

Female capable of passing recessive allele

Color blindness

• Males only need one recessive allele to have this

• Affects between 7% and 10% of caucasian males

• Bright greens seen as tans, and red as reddish brown

Sex-linked disorders

Carried on the X chromosome

X-linked recessive disorder

• More males than females will have the disorder

• The recessive allele on the x chromosome is always expressed in males

• The Y chromosome lacks an allele for the disorder

• Disorder are often passed from grandfather to grandson

  • Daughters of a male with the disorder are carriers

X-linked dominant disorder

• Only a few are known

• Affected males pass the trait only to daughters who have a 100% chance of inheriting the disorder

• Females can pass the dominant allele to both sons and daughters causing the disorder

Incontinentia pigmenti

Example of a X-linked dominant disorder

Duchenne muscular dystrophy

• Characterized by wasting away of muscles

• Symptoms include waddling gait, toe walking and frequent falls

• Caused by absence of protein dystrophin

Fragile X syndrome

• Caused by abnormal number of repeat sequences in the genome

• Most common cause of inherited mental impairment range from mild learning disabilities to more severe intellectual disabilities

• Associated with some forms of autism

• Males have characteristic physical abnormalities

• Less frequent and milder symptoms in females

Hemophilia

• Disease is caused by absence or minimal presence of a clotting factor

• Blood does not clot or clots slowly

• Bleed excessively after external injury, but also internally especially at joints

Hemophilia A and Hemophilia B

Two types of hemophilia

Red blood cells

Except for these, any cell can be a source of chromosomes for examination

Stains are applied and cells are photographed

Cells are stimulated to divide and a chemical is used to stop cell division when chromosomes are compacted

Karyotype

Chromosomes arranged by pairs according to their size and general appearance

Nondisjunction

• Failure of chromosomes or sister chromatids to separate during meiosis

• Can occur during meiosis I when both members of a homologous pair go into the same daughter cell

• Or it can take place during meiosis II when sister chromatids fail to separate and both daughter chromosomes go into the same gamete

Trisomy or monosomy

Results of gametes having incorrect number of chromosomes

Trisomy

Chromosome present in 3 copies

Monosomy

Chromosome present in 1 copy

Trisomy 21

Down syndrome, has a reasonable chance of survival after birth

Chromosome 21

One of the smallest chromosomes

Trisosomes

Tolerated better than monosomies

Trisomy 13 and 18

Rarely survive past first few months of life

Down syndrome

• Most common autosomal trisomy among humans

• Easily recognized physical features

• Chances of having children with this syndrome rapidly increases with age, starting at about age 40

• Extra chromosomes occur in egg or sperm

  • Some symptoms may be due to extra copy of gart gene on the long arm of chromosome 21

  • Causes high level of purines in blood and mild to severe mental impairment

• Can by diagnosed through karyotyping

Abnormal sex chromosome number

Caused by inheriting too many or too few X or Y chromosomes

Nondisjunction during oogenesis or spermatogenesis

Results in gametes with too few or too many X or Y chromosomes

Presence of Y chromosome

Is the determining factor for maleness not the number of X chromosomes

SRY gene

Produces a hormone, testis determining factor, plays a key role in male genital development

Can survive

Zygote with one chromosome

Barr body

• An inactive x chromosome

• In normal xx females, one of the x chromosomes is inactivated and becomes a darkly stained mass

Females

Can function with a single x chromosome, like males

Turner syndrome

• Females that have one x chromosome

• Usually are short with broad chest, and folds of the skin on the back of the neck

• Most have ovarian failure and do not undergo puberty or menstruate without hormone therapy, and their breasts do not develop

Klinefelter Syndrome

• A male with two x and one y chromosome (XXY)

• Testes and prostate gland are underdeveloped and facial hair is lacking, may have some breast development

• large hands and feet

• long arms and legs

• Slow to learn but do not have an intellectual disability unless more than 2 x chromosomes are inherited

• Assisted reproduction to have children

Poly-x female

Have more than two x chromosomes and extra barr bodies

Triplo-x

• Three chromosomes

• No distinctive phenotypes, except taller and thinner

• Sometimes delayed motor and language development

• Sometimes menstrual difficulties but most are fertile

Poly- x four chromosomes

• Have 4 x chromosomes, more rare

• usually tall

• Tend to have severe intellectual disability

• Physical abnormalities but menstruate normally

Jacobs syndrome

• XYY males resulting from nondisjunction during meiosis II or spermatogenesis

• Males are taller, acne, speech and reading problems, fertile and can have children

• No behavioral differences between XYY and XY males