Human Genetics
Modified PPT from K. Nifong
Reproduction
• Asexual
• Produces identical copies
• Mitosis
• Binary fission (in prokaryotes)
• Usually faster
• Sexual
• Requires two gametes each
carrying half of the chromosome
set
• Egg and sperm
• Creates variation
• Meiosis
Meiosis Review
• Key points:
• Haploid vs. Diploid
• Meiosis leads to gamete (haploid, 23 chromosomes, n)
• Fertilization leads to zygote (diploid, 46 chromosomes, 2n)
• Occurs in two steps: meiosis I and meiosis II
• Video: https://www.youtube.com/watch?v=XROMXTrklmQ
Where does variation come from?
• Independent assortment of alleles/chromosomes into haploid
gametes
• Crossing over during meiosis prophase I
• Fertilization (random combination of two chromosome sets)
Studying genetics using a Karyotype
• Karyotype
• A chart of chromosome pairs that is used in pinpointing unusual chromosome
numbers in cells
• The human karyotype includes 23 sets of chromosomes.
• Chromosomes 1-22 are autosomes
• Chromosomes #23 are sex chromosomes
5
The Human Karyotype
6
Mistakes in meiosis: chromosome issues
• Aneuploidy
• The condition of having an abnormal number
of chromosomes
• Examples:
• Trisomy…down syndrome …21st chromosome
• Usually results from nondisjunction
7
Mistakes in meiosis: chromosome issues
Nondisjuction: When chromosomes fail to separate correctly
Both chromosomes from a homologous pair move to the same pole of the cell
Trisomy: One form of nondisjuction where zygote has an extra chromosome
Ex: Trisomy 21
8
Mistakes in meiosis: chromosome issues
Monosomy: A form of nondisjuction where zygote is missing one chromosome
In humans, most zygotes/organisms with monosomy do not survive
Ex: (non lethal) Turner Syndrome= Human females with only one X
chromosome
9
Mistakes in meiosis: chromosome issues
Nondisjuction of homologous chromosomes
Complete Diploid set of chromosomes is passed to gamete
Offspring has 3 sets of chromosomes after fertilization
Triploid
Polyploids: Organisms with more than 2 sets of chromosomes
(Greater than Diploid)
Rare in animals and usually causes death
Occurs frequently in plants
Flowers/fruits are larger and usually healthier
10
Checkpoint #1
1. Why does sexual reproduction produce more variation? What
would be the evolutionary advantage of variation?
2. How does nondisjunction occur? What are the possible results?
3. If we were to look at a karyotype of your chromosomes, what
would it look like?
4. Why is meiosis vital to sexual reproduction?
5. Summarize the process of meiosis by describing what is present at
the start and at the end, and using the terms haploid, diploid,
gamete. (do not need to describe each step).
Inheritance within the chromosome: genes
• Autosomal traits: carried on autosome (body) chromosomes
• Can be dominant or recessive
• Sex-linked traits: carried on the X sex chromosome
Dominant Autosomal Heredity
http://ology.amnh.org/genetics/geneticjourney/images/curlTongue.gif
• Examples:
http://www.fremont.oxnardsd.org/genetic/images/ears.gif
• Tongue curling
• earlobe type
• attached (recessive)
• unattached (hetero. Or homozygous dominant)
• Huntington’s Disease
• Nervous system degenerates
• Usually occurs between the ages of 30 to 50
•
polydactyly
• Dominant (more than 5 fingers or toes)
http://images.google.com/imgres?imgurl=www.thecat
gallery.com/images/polydactyl-feet4.JPG&imgrefurl=http://www.thecatgallery.com/polyd
actyl_cats.html&h=205&w=250&sz=9&tbnid=FNmfWE
wiZYcJ:&tbnh=86&tbnw=104&prev=/images%3Fq%3D
polydactyl%2Bfoot%26hl%3Den%26lr%3D%26ie%3DUT
F-8%26oe%3DUTF-8
13
Recessive autosomal heredity
• Examples:
• Cystic fibrosis(CF)
• Mucus thick in lungs and digestive
tract…collects in lungs…breathing difficult
• Sickle cell anemia
• Red blood cells with sickled shape and has
shorter life span than normal red blood
cells
• Leads to anemia and poor circulation
• Tay–sachs disease
• Affects central nervous system
• Leads to blindness, loss of movement and
mental deterioration
http://www.defiers.com/sc.jpg
http://images.google.com/imgres?imgurl=www.nlm.nih.gov/medli
neplus/ency/images/ency/fullsize/18135.jpg&imgrefurl=http://ww
w.nlm.nih.gov/medlineplus/ency/imagepages/18135.htm&h=320&
14
Recessive autosomal heredity
• Phenylketonuria (PKU)
• Caused by the absence of the enzyme that converts
phenylalanine to tyrosine
• Affects central nervous system
• Can cause mental retardation if not treated at birth
• Treated by changing diet
http://images.google.com/imgres?imgurl=www.nlm.nih.gov/
medlineplus/ency/images/ency/fullsize/2961.jpg&imgrefurl=
http://www.nlm.nih.gov/medlineplus/ency/imagepages/296
1.htm&h=226&w=306&sz=9&tbnid=ZkaiRonP1jwJ:&tbnh=82
&tbnw=111&prev=/images%3Fq%3DPhenylketonuria%26hl%
3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26sa%3DN
Blood is drawn when babies are born to test for PKU
and blood type
15
Sex Linked Traits
• Humans have 22 pairs of autosomes, or chromosome
pairs that resemble one another. The 23rd pair of
chromosomes are called the sex chromosomes.
• Sex Linked Traits: Traits controlled by alleles that are
found on the sex chromosomes, usually on the X
chromosome.
• Ex. Color blindness, hemophilia (See p. 338).
• Notation is XBXb or XBXB (female) and XBY or XbY (male) instead
of BB, Bb or bb.
• Can a man pass on a sex linked trait to his son?? Show work.
16
The Human Pedigree
• A way of showing a family tree that helps us understand
diseases and where the dangerous alleles exist within a
family.
• A circle is a female
• A square is a male
• Dark color is an affected individual
or
• ½ dark and ½ light is a carrier, or an individual that does
not show the phenotype but carries the dangerous, often
lethal allele.
• Slash is death.
17
The Human Pedigree
18
Checkpoint #2
1. Update your family tree by drawing it in the form of a pedigree.
2. When looking at a karyotype, how do you know if an individual is
male or female?
3. What does it mean for a disease to be “autosomal recessive”?
4. Draw a Punnett square showing a cross between a normal male for
color-blindness and a carrier female.
5. I once had a cat that was polydactyl. Since that is a dominant
autosomal trait, would all of his kittens be polydactyl? Why or why
not?