Download Mendelian Genetics

Mendelian Genetics
GREGOR MENDEL: 1860, father of genetics. An Austrian monk who
developed the laws of inheritance by studying garden peas. Showed that
parents can pass heritable factors to their children (genes).
 Genes: stretches of DNA on chromosomes that can determine a trait.
Always located on the same spot or locus on a chromosome.
Genes have alternate forms called alleles, and every organism has 2
alleles for every trait that are found on homologous chromosomes
(one from mother, one from father).
 Dominant allele: form of the trait that will always be expressed. use
capital letter to denote it.
 Recessive allele: form of the trait that will be masked or overtaken by
the dominant trait. Use lower case letters to denote it. need two copies
of the allele to express it.
 Homozygous: have 2 copies of the same allele. Ex: BB, bb
 Heterozygous: one copy of each alternative allele. Ex: Bb
 Codominant alleles: alleles that are expressed equally. Have the same
dominance.
 Incompletely dominant alleles: one allele is not completely dominant
over the other. The phenotype is somewhere in between or an
intermediate between both traits.
Principles of Segregation: Mendel performed experiments where he
followed the inheritance of one trait only. This is called a monohybrid cross
because the parent plants differ in one characteristic only.
- Parental generation is called P generation.
- The offspring of the P generation are the F1 generation.
- The offsprings of the F1 generation are the F2 generation.
Used purple flowers versus white flowers on pea plants.
Mendel deduced from his experiments:
1. there are 2 forms of each trait called alleles
2. the organism inherits one allele from the mother and one from the father
3. allele pairs segregate during meiosis, each sperm and egg (gamete) carry
one allele for each inherited trait
4. Fertilization gives the offspring 2 alleles. alleles can either be dominant
or recessive.
Use PUNNETT square to solve a genetic problem. keep track of the
gametes.
 figure out the genotype of the parent
 figure out possible gametes
 use the square to determine all possible genotypes and phenotypes of
the offspring.
From the punnett square, Mendel showed that the generation of F1 will all
be purple with a Pp genotype, and that F2 will have 3/4 purple and 1/4
white flowers. genotype: PP, 2Pp, pp
F1 generation:
p
p
P
Pp
Pp
P
Pp
Pp
 All F1children are heterozygous
 this is always the case if you mate a homozygous dominant with a
homozygous recessive gene
F2 generation: Pp x Pp
P
p
P
PP
Pp
p
Pp
pp
 3/4 have purple phenotype but one is homozygous ( PP) and 2 are
heterozygotes (Pp)
 1/4 are white, homozygous (pp)
 phenotype ratio is 3:1 purple vs. white
 genotype ratio is 1:2:1 ( 1PP, 2 Pp, 1pp)
 this is always the ratios for a heterozygous monohybrid cross
The principle of independent assortment is shown by following 2 traits at
the same time.
It states:
- each pair of alleles segregates independently during gamete
formation
- all combinations can occur in the gametes
Dihybrid cross: mating of parents differing in 2 traits.
RY: round yellow seeds, dominant
ry: wrinkled green seeds, recessive
crossed RRYY x rryy -------> gametes RY and ry
F1 generation are RrYy ----> round and yellow peas. all offsprings are
heterozygous for one trait.
F2 generation: RrYy x RrYy-------> gametes RY, rY, Ry, ry
F2 generation:
RY
rY
Ry
ry
RY
RRYY
RrYY
RRYy
RrYy
rY
RrYY
rrYY
RrYy
rrYy
Ry
RRYy
RrYy
RRyy
Rryy
ry
RrYy
rrYy
Rryy
rryy
We have: 9 round yellow peas (dominant, dominant), 3 round green peas (
dominant, recessive), 3 wrinkled yellow peas ( recessive, dominant), 1
wrinkled green pea ( recessive, recessive). The ratio of phenotypes for a
dihybrid cross is always 9:3:3:1.
This behavior is attributed to the behavior of genes after meiosis. The alleles
segregate as the homologous chromosomes separate in anaphase I. This can
lead to many different combinations of gametes depending on how the
chromosomes align during metaphase I.
1/4 RY, 1/4 ry, 1/4 rY, 1/4 Ry
Testcross: use it to determine an unknown genotype. If you are not sure of
the genotype, homozygous or heterozygous for a trait, then cross it with a
homozygous recessive.
Two possibilities:
AA x aa
A
a
Aa
a
Aa
A
Aa
Aa
All dominant offspring ( heterozygous).
Aa x aa
A
a
Aa
a
Aa
a
aa
aa
1/2 dominant and 1/2 recessive. 1:1 ratio.
How are human traits inherited? We study the results of matings that have
already occurred and we track the inheritance of a trait over many
generations in a family.
Pedigree Charts or Trees: analyze inheritance of a disorder or a gene in a
family.
 males are squares
 females are circles
 affected individuals will have shaded circle or square, unaffected are
not shaded
 an individual who has one copy of the recessive allele is a carrier.
Incomplete Dominance: is a situation where one allele is not completely
dominant over another. so the result is somewhere in between the two traits
that are being studied. The heterozygote shows traits intermediate in
between the two homozygote parents.
Ex:
RR -----> red flowers
rr -------> white flowers
F1 generation: RR x rr ------> Rr pink flowers, intermediate color.
F2 generation: 1red(RR), 2 pink(Rr), 1 white(rr)
1:2:1 ratio.
ABO Blood Group: Some genes have multiple alleles, more than two
possible alleles exist fro the same gene. For the ABO blood group , there are
3 possible alleles, A, B, and O which can produce 4 phenotypes A, B, AB,
O. AB is an example of codominance in genes.
A: RBC have carbohydrate A
B: RBC have carbohydrate B
O: RBC have no carbohydrates
Rh factor: Rh +ve or Rh -ve, is a separate carbohydrate that is inherited
separately than the ABO system.
Polygenic Inheritance: some traits are influenced by many genes. Example
skin color, height, eye color. This occurs when genes have an additive effect
on the phenotypic appearance of a person.
skin color has at least 2 pairs of alleles
very dark dark medium
light v.light
AABB
AABb
AaBb
Aabb aabb
AaBB AAbb
aaBb
aaBB
Many inherited disorders are controlled by a single gene.
Recessive Autosomal Disorders:
Usually born to parents that are normal but heterozygotes for the trait.
Carriers.
1. Albinism: lack of pigment in skin, hair, and eyes. sensitive to light.
2. Tay-Sachs Disease: common in jewish people, lipid accumulation in
brain, development slows down , blind, seizures, and paralysis.
3. Cystic Fibrosis: excessive secretion of tick mucus which interferes with
breathing, digestion, and liver function. gene on chromosome 7. Untreated
life expectancy is about 5 years.
Dominant Autosomal Disorders: the presence of one allele is sufficient to
show the phenotype. heterozygote.
1. Achondroplasia: dwarfism, head and torso are normal but arms and legs
are short. homozygous genotype causes fetal death.
2. Huntington's Disease: neurological disorder, mental deterioration,
depression, no swallowing or speaking. death in 10-20 yrs.
Many genetic disorders can be detected before birth.
1. Amniocentesis: sample of amniotic fluid is examined by Karyotyping to
check for genetic disorders. cells need to grow in culture for a while before
they are tested. 14 - 20 wks of pregnancy. Ex: Downs Syndrome.
2. Chorionic Villus Sampling(CVS): a tube is inserted in cervix and sucks a
little of fetal tissue from the placenta. 10-12 wks. Karyotyping can be
carried out immediately.
3. Ultrasound: direct examination of the fetus by sound waves. Can get
enhanced and detailed images of the spine, head, feet, hands, toes, and
major organs.
Linked Genes: genes that are located close together on the same
chromosome tend to be inherited together. These genes do not follow the
Mendelian pattern of independent assortment. The genes are inherited
together on adjoining portions of the chromosome. This pattern is similar to
the 3:1 ratio of monohybrid cross rather than the 9:3:3:1 ratio of a dihybrid
cross. Sometimes, crossing over occurs between homologous chromosomes
which leads to the production of new alleles not found in the parents.
Sex Chromosomes: most chromosomes in cells are not sex chromosomes (
1 sex chromosome), they are called autosomes ( 22 pairs autosomes).
X and Y are in humans, fruit flies
X and O in grasshoppers, crickets.
Z and W in birds, fish, butterflies.
 XX individuals are females. All the eggs contain one X chromosome
in them.
 XY individuals are males. Sperm can either carry an X or a Y
chromosome.
 If an X sperm fertilizes an X egg, a female fetus develops. If a Y
sperm fertilizes an X egg, a male fetus develops.
 There are other genes on the sex chromosome that do not determine
sex. These genes are called sex-linked genes.
 Mainly these are found on the X chromosome
 Males usually inherit this from their mothers only, since they get their
X chromosome from their mothers.
 males usually need only 1 copy of the allele to express it, while
females require 2 copies of the recessive sex allele to be affected.
 Women can be carriers for a sex linked allele if they carry it on one of
their x chromosomes. They can pass it on to their sons or have carrier
daughters.
 Ex: color blindness, hemophilia, Duchenne muscular dystrophy
Red green color blindness: carried on the X chromosome.
XBXB = female normal vision
XBXb = carrier female with normal vision
XBY = normal male
XbY = color blind male
Hemophilia: sex linked recessive gene that prevents normal blood clotting
which may cause excessive bleeding after a minor cut. need clotting factors
and blood transfusions.
XHXH = female, normal clotting
XHXh = female, carrier for recessive gene
XhXh = hemophiliac female
XHY = normal male
XhY = hemophiliac male
Duchenne Muscular Dystrophy: sex linked recessive disorder
characterized by the progressive weakness of muscles. Eventually all the
muscles in the body become paralyzed and breathing becomes difficult.
Death occurs around age 20. Cases occur in males.
Mistakes can occur during meiosis that can alter the number of
chromosomes in the gametes produced in our ovaries and testes.
Nondisjunction: members of a chromosome fail to separate leading to
abnormal numbers of chromosomes.
Sometimes gametes can have the wrong number of X and Y chromosomes.
Either 2 X's or 2Y's or one is missing.
XXY: Klinefelter syndrome: sterile males with abnormally small testes.
long arms and legs, some breast enlargement, slow but not mentally
retarded.
XYY, Jacobs syndrome: taller than average, may have some reading and
speech problems.
XXX, Metafemales: no physical abnormality, some learning disabilities,
limited fertility.
XO, Turner syndrome: short stature, web of skin from the neck, sterile,
normal intelligence.