Download Human Genetics

Human Genetics
Chromosomes
• Genes are found on chromosomes
– Chromosomes are rod-shaped structures located
in the nucleus of every cell in an organism.
• Control all the traits of an organism.
– Sex cells are produced during a process called
meiosis “Me”.
• Only half the information of an organism can be
handed down to an offspring.
Nucleus Chromosome
– Parent = 46 chromosomes
– Sex cell = 23 chromosomes
Chromatid
Centromere
Cell
Chromatid
Mitosis vs. Meiosis
4
Body
Cells
4
Sex Cells
“Me”
“My toes”
8
8
4
4
4
4
2
2
2
2
Karyotype
• A picture of the 23 chromosome pairs a
person has.
– Can be used to identify the sex of an individual or
see if an individual has any abnormalities.
Normal
Karyotype
23
Sex Chromosomes
• The 23rd set of chromosomes are called the
sex chromosomes.
– Determine the sex of the offspring:
• Male = XY
• Female = XX
23
Determining Gender
Mom = XX
Dad = XY
X
Y
X
X
XX
XX
XY
XY
Genotype:
50% - XX
50% - XY
Phenotype:
50% = Female
50% = Male
Sex-Linked Traits
• All the genes on the X or Y chromosome are
inherited together.
• This means that certain traits are always
inherited with certain sex chromosomes.
• Examples:
– X-linked traits
• Color-blindness
• Hemophilia
– Y-linked traits
• Hairy ears
X-Linked Traits
• Color Blindness – Inability to distinguish
different colors, usually reds and greens.
– Recessive trait (Xb), Normal vision (XB)
– Inherited on the X chromosome
– More common in males. Why?
• You only need one affected X chromosome to inherit the
trait. (XY)
• Females can inherit one affected X chromosome but not
show the trait (XX)= carriers
Males
Females
XBY - normal
XbY – color blind
XBXB – normal
XBXb – carrier but normal
XbXb – color blind
Color Blindness
Xb
Xb
Mom = XbXb
= Colorblind
Dad =
XBY
= Normal
Genotypes:
50% - XBXb
50% - XbY
XB
Y
XBXb
XBXb
XbY
Xb Y
Questions:
What percentage of children are carriers, who? _50%_- females
What percentage of children are color blind, who? 50%_- males
Can males ever be carriers, why? NO – only receive 1 X
What percentage of the children are normal? 50%
X-Linked Traits
• Hemophilia – blood clotting disorder.
– Recessive trait (Xh), Normal clotting (XH)
– Inherited on the X chromosome
– More common in males. Why?
• You only need one affected X chromosome to inherit
the trait.
Males
XHY - normal
XhY – hemophiliac
Females
XHXH – normal
XHXh – carrier but normal
XhXh – hemophiliac
Hemophilia
Mom =
XHXh
XH
Xh
XHXH
XHXh
= carrier
Dad = XHY
XH
= Normal
Genotype:
25% - XHXH
25% - XHXh
Y
XHY
XhY
25% - XHY
25% - XhY
Questions:
What percentage of the children are carriers, who? _25% - females
What percentage of the children are hemophiliacs, who? 25% - males
What percentage of the children are normal? 75%
Y-Linked Traits
• Hairy Ears – a trait handed down on the Y
chromosome that produces men with hairy
ears.
– Why not hairy eared women?
• Women don’t inherit Y chromosomes!
Males
XYH - normal
XYh – hairy ears
Females
XX – normal
Hairy Ears
X
X
X
XX
XX
Yh
XYh
XYh
Mom = XX
= normal
Dad = XYh
= hairy eared
Genotype:
50% - XX
50% - XYh
Questions:
What percentage of the children are carriers, who? 0%
What percentage of the children are hairy eared, who? 50% - males
Can females have hairy ears, why? No – no Y chromosome
Pedigree Charts
• Another model for understanding inheritance is a
pedigree chart.
– Each level represents a new generation.
• Lines connect offspring to parents
– Males are squares and females are circles.
– Blank squares or circles usually represent individuals who do not
carry a recessive trait
– Shaded in squares or circles represent individuals who carry
recessive traits.
– Circles that are partially shaded in represent carriers, females who
carry a recessive trait but do not show the trait and would
therefore be considered normal.
1
2
3
4
Questions:
• What is the Mom considered in the Parent generation? Carrier
• How many offspring did the parents have? 4
• How many of the offspring, in the second generation, are:
• Normal, who? 3
• Carriers, who? 1 – female
• Colorblind, who? 1 - male
Nondisjunction – “Not Coming Apart”
• Nondisjunction – the failure of chromosome pairs to separate
properly during cell division.
– Result is a cell with an imbalance of chromosomes.
Kleinfelter’s Syndrome
• 47, XXY Syndrome – males have an extra X chromosome.
• Most common sex chromosome disorder.
• Characteristics:
– Smaller testicles
– Reduced fertility
Karyotype
Down Syndrome – “trisomy 21”
• 3 copies of chromosome 21
• Characteristics:
–
–
–
–
Mental & developmental disabilities
Almond-shaped eyes
Deep crease on palms of hands
Short limbs
Mutations
• Mutation – A change in a gene or chromosome.
– If in a body cell, it affects only the organism carrying
it.
• Example: Cancer
– If in a sex cell (egg, sperm), mutation can be passed
on to an offspring.
• Found in 3 ways:
– Positive
– Negative
– Neutral
Positive Mutation
• Positive – Benefits the organism
– Example: Mutation causes disease resistance
• Plants that can resist disease or drought
• Humans that can resist HIV infection
• Mutations that cause pleasing new feather colors in
birds
Neutral Mutations
• Neutral – Does not benefit or harm the
organism
– Example: Extra toes and fingers
– Example: White skin
Negative Mutations
• Negative – Harms the organism or reduces its
chances of having offspring
– Example: Sickle-cell anemia
• Often in people of African descent
• Blocks blood vessels, causes pain,
strokes, death
• Can also be a positive mutation
– Africa – lots of malaria
– One copy of the mutant gene
protects people from malaria