Chapter 11 &14

Human Geneticsand Meiosis

The study of inheritance patterns in humans

Oculocutaneous albinism

What each of the human chromosomes look like

Karyotype: A photomicrograph of chromosomes arranged according to a standard classification

In other words…

• Chromosomes are digitally arranged so that they are matched with their homologue or “partner” chromosome.

• Homologue chromosomes are the same size, shape, and carry the same genes, and one is inherited from each parent.

• They are numbered according to size.

Sex determination with karyotype

• This karyotype has 23 exact pairs, which means the person is female.

• Note that #23 chromosomes are both X.

Normal human male

• Note that #23 chromosomes are X and Y.

Is this person female or male?

Trisomy 21

• Abnormality shown in karyotype

• Note that there are three copies of #21 chromosome.

• This person has Down Syndrome.

Photos of Down Syndrome patients from the National Down Syndrome Society

Correlation between mother’s age and Trisomy 21 incidence

Monosomy X

• Abnormality shown in karyotype

• Note this person only has 1 copy of the X chromosome.

• This female has Turner’s syndrome.

XXY Male (Extra X)

How are DNA samples obtained for karyotypes?

Amniocentesis: obtaining amniotic fluid which has cells from the fetus

Chorionic villi sampling: removing cells from the chorion with fetal tissue

If there are chromosomal number abnormalities, how do they form?• Meiosis: the process of creating sperm or

egg from a diploid cell

• If there is a mistake when chromosomes are separating, then the resulting sperm or egg will have too many or too few chromosomes.

Click on image to play video.

Meiosis 1

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Oocyte or Spermatocyte

• This cell that can undergo meiosis originally has 6 chromosomes and has replicated to 12 chromosomes in preparation for meiosis.

Prophase I: homologous chromosomes pairing into tetrads

Metaphase I: tetrads align, along the metaphase plate

Anaphase I: homologous chromosomes separate from the metaphase plate

• If chromosomes do not properly separate, this is called nondisjunction.

• Nondisjunction leads to trisomy and monosomy disorders.

Telophase I: membranes form around the separated homologues

Prophase II: spindle fibers bind to the sister chromatids of each chromosome

Metaphase II: chromosomes align along the metaphase plate

Anaphase II: sister chromatids separate to opposite poles

Telophase II: nuclear membrane forms around newly separated chromatids

• Note that each new nucleus formed has ½ the amount of DNA as the original cell.

• These cells are haploid cells.

Nondisjunction

How can siblings look alike but not exactly the same if they come from the same parents?

Crossing over

• The chromosomes during prophase I undergo crossing over, where parts of the homologues randomly switch places.

Importance of crossing over

• The gene combinations that a person gets from his or her parents will be different, to varying degrees, than the combination a sibling may get.

More sibling similarities

What other chromosomal disorders can arise?

• Deletion• Inversion• Translocation*• Duplication

*Don’t worry about this one.

Chromosomal mutations

Deletion

Duplication

Inversion

Translocation

Problems with chromosomes

• Duplication: copied parts of chromosome

A

B

C

D

A

B

B

B

C

D

Problems with chromosomes

• Deletion: missing parts of chromosome

A

B

C

D

A

D

Problems with chromosomes

• Inversion: parts of chromosome tched

A

B

C

D

A

C

B

D

Human genetic disorders from deleterious genes

• Sometimes the alleles inherited contribute to disorders and not from the number or shape of the chromosomes.

1. Sex-linked: genes found on X or Y chromosome

2. Recessive: requires 2 allele copies to express disorder

3. Dominant: requires only 1 allele copy to express disorder

Recessive disorders

Dominant disorder

Sex-linked disorder

Are you red-green color blind?

• Yes, if you have a difficult time distinguishing a number from this picture

Pedigrees: a chart which can show trait inheritance through several generations

Albinism

Symbols

Male Female

marriage

More symbols

She gave birth!

Pain! Suffering!

Even more symbols

… and they have a litter of 4!

Still more symbols

Affected. AA or aa

Normal heterozygous

Albinism – recessive disorder

aa Aa

Aa aa Aa Aa

Deafness – dominant disorder

Aa or AA?

Aa Aa Aa Aa

aa

Mother must be AA.

Colored blindness – Sex linked

XCXc

XcY XCXc XCY XcXc

XcY

Pedigree of Hsu family

Me!

Review Videos

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