Transcript Human Genetics Unit - Delsea Regional High School

Human Genetics
Unit
Creating Genetic Diversity
How did you become so unique?
1) Sexual reproduction
2) Independent assortment of
chromosomes (from mom and
dad)
3) Random fertilization of gametes
Lets think about this
Egg and Sperm with 223 (or about 8 million)
different combinations of chromosomes
are produced through meiosis
Sexual reproduction involves egg and sperm
A sperm (with 223 different chromosome
combinations) can fertilize an egg (with
223 different chromosome combinations)
Any sperm can fertilize any egg
Lets think about this
So what does all that mean?????????
You are the product of 223 x 223 (or more
than 70 trillion) different combinations of
chromosomes
Or
You are the outcome of 1 in 70 trillion
(70,000,000,000,000) different
combinations of chromosomes from mom
and dad
Creating Genetic Diversity (cont)
4) Crossing over
= the exchange of chromosome
segments between homologous
chromosomes
- occurs during meiosis
1) Two homologous
chromosomes pair up
with each other during
prophase I of meiosis.
2) In this position, some
chromatids are very
close to each other and
segments cross.
3) Some of the
segments break off and
reattach to other
chromosomes
Gene Linkage
= genes located close together
tend to be inherited together
- discovered by American
scientist Thomas Hunt Morgan
- studied fruit flies
Genetic Linkage
Crossing over is more likely to happen
between B and C, separating A &B from
C & D.
Gene Linkage (cont)
- Morgan’s Conclusions
1) Linked genes are on the
same chromosomes
2) The farther apart two genes
are located, the more likely
they are to be separated by
crossing over (and vice versa)
Genetic Linkage
Crossing over is more likely to happen
between B and C, separating A &B from
C & D.
Gene Linkage (cont)
- linkage maps show relative location
of genes on a chromosome
Polygenic Traits
= traits produced by two or more
genes
- will show a range of possibilities
(ex: skin color, eye color)
Polygenic Traits (cont)
- epistasis = when one gene
interferes with the expression of
other genes
- ex: albinism
Other Albino Animals
Environment Influence on
Phenotype
- Are identical twins always
IDENTICAL?
No
- nutrition, health care, money,
disease, injuries
Environmental Influence on
Phenotype (cont)
In animals:
- Light – controls the color
expression in some insects
Environmental Influence on
Phenotype (cont)
In animals:
- Temperature
- controls the sex of certain types
of reptiles
Environmental Influence on
Phenotype (cont)
In animals:
- Temperature
- controls coloration in some mammals
Environmental Influence on
Phenotype (cont)
- External alteration of
phenotype does not affect
genotype
- your phenotype is a mixture
of your genes (genotype) and
the environment you live in
Sex Determination
- Male = XY
Female = XX
- normally, 50% of the offspring
of an organism born are male,
50% are female
Sex Determination (cont)
Genotypic Ratio
Phenotypic Ratio
Sex-Linked Traits
= traits caused by genes
located on the sex
chromosomes
- most genes that determine
your phenotype are found on
autosomes, however, some
are found on sex
chromosomes
Sex-Linked Traits (cont)
- most sex-linked traits are found
on the X chromosome
- males are more likely to show a
sex-linked trait or disorder than
females
- Why? Because males only have
one X chromosomes
Sex-Linked Traits (cont)
- Example 1:
Red eyes are dominant to white
eyes in fruit flies. Cross a
homozygous dominant red-eyed
female with a red-eyed male. Eye
color is a X-linked trait.
Example 1
X
Y
X
X X X Y
X
X X X Y
Genotypic Ratio
2 XRXR: 2 XRY
Phenotypic Ratio
2 red-eyed females:
2 red-eyed males
Sex-Linked Traits (cont)
- Example 2:
Red eyes are dominant to white
eyes in fruit flies. Cross a
heterozygous female with a red
eyed male. Eye color is an Xlinked trait.
Example 2
X
Y
X
X X X Y
X
X X X Y
Genotypic Ratio
1 XRXR: 1 XRXr: 1 XRY: 1 XrY
Phenotypic Ratio
2 red-eyed females:
1 red-eyed male:
1 white-eyed male
Sex-Linked Traits (cont)
- carrier = does not show a
disease/disorder but can pass
it onto their offspring
- aka: heterozygous
Genetic Disorders
1) Single allele disorders =
controlled by a single allele
(letter) of a gene, either dominant
or recessive
- genetic marker = a short section
of DNA that is known to have a
close association with a particular
gene located nearby
Cystic Fibrosis - Recessive
Huntington’s Disease - Dominant
Genetic Disorders (cont)
2) monosomy = a zygote with
45 chromosomes
- has only one copy of a
particular chromosome
Genetic Disorders (cont)
3) trisomy = a zygote with 47
chromosomes
- has three copies of a particular
chromosome
- most common example is down
syndrome (aka: trisomy 21)
- has an extra copy of
chromosome 21
Genetic Disorders with the
Sex Chromosomes
-
XO = Turner’s Syndrome
XXY = Klinefelter Syndrome
XXX = Triple X Females
XYY = Jacob Syndrome
YO = Does not exist…need an X
chromosome to live
What about hermaphrodites?