Download Genetics - Crestwood Local Schools

Genetics! - Mendel and Heredity!
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Heredity - the transmission of traits from parent to offspring
Genetics - study of heredity
* Research done by Gregor Mendel (1822 - 1884)
-began research by studying pea plants and noting traits
that were passed down through generations
- he went further by counting the # of plants with certain
traits in each generation
- led to today's way of research
Why peas?
1.) Many varieties exist - easy to tell the difference between
them
2.) One of the 2 forms of each trait would disappear in a
generation and then reappear in the next - easy to count this
3.) Fast growing and produces many offspring
4.) Easy to cross-pollinate
*Note* peas are self-pollinaters - they don't need
another flower to reproduce
- Mendel removed the male parts of one flower (stamen)
and removed the female parts from another (pistil ). He used the
opposite to fertilize the other flower. Cross-pollination
Ex.
Mendel's experimental design:
3 steps…
1.) Allowed each pea plant to self pollinate for several generations
- this ensured true-breeding - offspring only displays one
form of a trait
* P generation = parent generation
2.) Cross-pollinated 2 P gen. that had contrasting traits
Ex: purple flowers with white flowers
* F1 generation = first filial gen.
Noticed all flowers of the F1 gen. were purple!
3.) Allowed F1 gen. to self pollinate one time
*F2 generation = second filial gen.
*These are the plants that he counted!
Ex:
Observed 2 ratios:
Dominant traits - trait that remains seen (expressed) in F1
gen.
Recessive traits - trait that is not seen (not expressed) in F1
gen.
He studied 7 traits:
* Flower color, seed color, seed shape, pod color, pod shape,
flower position, and plant height.
** Pg. 264 in book - Fig. 11-3 **
Observed a 3:1 ratio in F2 gen. every time!
*Noticed that the recessive plants were true breeding when
allowed to self-pollinate.
*Also noticed that the dominant plants were producing 3:1 ratio
when they self-pollinated!
**This meant the 3:1 ratio in F2 was really 1:2:1!
1 plant = dominant , true-breeding
2 plants = not true-breeding
1 plant = recessive , true-breeding
This led to a Theory of Heredity…
5 parts:
1.) Traits are passed on to offspring through genes
2.) For each trait, you get one gene from Mom, one from Dad
*each gene may not have the same info!
Same info = homozygous
Different info = heterozygous
*Each copy of a gene is called an allele
3.) Phenotype - how a trait looks when expressed
- determined by alleles that code for that trait
Genotype - the set of alleles that an individual has
4.) Each individual receives one allele from Mom, one from Dad
- can then pass on one of those alleles when they produce
offspring
5.) Just because you have an allele, doesn’t mean it will be
expressed
- in a heterozygous situation, only the dominant allele will be
expressed
This theory became the Law of Segregation - the members of each
pair of alleles separate when gametes are formed
ie: when sex cells form, the alleles separate
Another observation turned into the Law of Independent Assortment
-pairs of alleles separate independently of one another during
gamete formation
ie: when the alleles separate, they do so randomly
Probability and Punnett squares!
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When we look at the alleles, we use symbols to represent the
traits…
*CAPITAL LETTERS = DOMINANT
*lower case letters = recessive
**Must use the same letter to represent the two forms of one gene!
Ex: For height, you might use T for the dominant tall and
t for recessive short.
Each trait has 2 alleles so you must write the letter for each
allele!
Ex: homozygous dominant = TT
homozygous recessive = tt
heterozygous = Tt
*ALWAYS write dominant first for each trait!
Probability:
-the likelihood that a certain event will occur
Can determine probability like this…
Probability = # one kind of possible outcome
total # of all possible outcomes
Q: If there are 20 pea plants being tested for height, and 15 of
them had the dominant tall height; 5 had the recessive short
height… what is the probability of being a short pea plant?
A:
P = 5/20 ----- 1/4 So, there is a 1 in 4 chance the plant will be
short!
Monohybrid crosses:
- cross that provides data about one pair of contrasting traits
*both parents are homozygous for their trait
Ex: one is TT
one is tt
* can also both be heterozygous for their trait
*both are Tt
* or can be mixed
ex: one is TT
one is Tt
Can use a Punnett square to predict the probable outcome of a
cross!
The ratios are very different from each other depending on what
you are crossing…
Ex: TT x tt
Dihybrid cross:
Tt x Tt
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- involves 2 pairs of contrasting traits
Ex: plant that is homozygous for round yellow seeds x plant is
homozygous for wrinkled green seeds
*Notice - whenever you write more than one trait, there are certain
rules that will be followed:
~ keep the alleles for each trait together and write the
dominant first for each trait
~ when you separate them for the punnett square, put only one
allele per trait along the side and top of the square
~ keep the traits in the order they appeared in the cross
~ when you put them back together, match the letters up with
like letters so they the alleles are paired again
Ex: RRYY x rryy
What is the phenotypic ratio? Genotypic ratio?
Can also cross heterozygous traits like this…
RrYy x RrYy
Also try:
RrYY x rrYy
What are the ratios?
P:
G:
P:
G:
Incomplete dominance:
- both traits are shown as a mix or blend of the two traits
Ex: a white flower x a red flower = pink flowers in F1 gen.
Codominance:
- two dominant alleles are expressed at the same time
Ex: homozygous red horse x homozygous white horse
= a roan horse
- both red and white hairs are found on the horse's coat
Multiple alleles: (see pg. 273 for more examples)
- traits that have genes with more than 2 alleles
Ex: blood types A B O
A and B are dominant -- O is recessive
A and B can be codominant to form the AB blood type
So… can have 4 different blood types with many combinations
AA, Ao
BB, Bo
AB
oo
**Note with mult. alleles, you may see them written like this:
IAi or
IAIB
this just means the allele is carried on that particular chromosome
Polygenic traits:
*traits controlled by more than one gene are polygenic
ex: skin color in humans - more than 4 genes code for skin
color
eye color - at least 3 genes code for color but there may be
more
~ generally, brown is dominant to green which is
dominant to blue
*All poygenic traits are complex - if you would like to research
more, please do so for extra credit!
Environmental factors:
*some genes are triggered to work in different conditions
Ex: arctic foxes and rabbits : white fur in winter, brown fur in
summer
Human genetics!
C-14
We know that a mutation is a change in genetic material…
we will explore some genetic disorders that can result!
Two phenotypically normal people can carry a genetic disorder on
recessive genes and produce a homozygous recessive child in
which the disorder will show.
Tracking traits in families:
- can determine your pedigree (family history) by noting the traits
your family members show or have!
1.) determine if the trait is sex-linked or autosomal
sex-linked = seen only in males
autosomal = same for both
females and males
2.) determine whether it is dominant or recessive
dom = everyone with that gene will have the trait
rec = only those with both recessive genes will show it
3.) determine if the trait is dependent on one gene or many
one gene = children should have it in a 3:1 ratio
many genes = ratio much lower
Here's how a pedigree works:
circles are female
squares are male
shaded in means they express the trait
not shaded means they do not express the trait
~sometimes for genotypic pedigrees they will shade a half to
represent a carrier of a trait.
Ex:
~horizontal lines connecting male and female indicates marriage
~vertical lines or brackets indicate their children
How do you know if you might have a disorder or pass one to your
kids?
*Look at your pedigree for any disorders
*Can have an amniocentesis or chorionic villi sample done while
pregnant
In some cases, therapy may help those with a disorder
Here are some disorders that may be tracked using a pedigree...
Sickle cell anemia:
- mutated allele that produces a defective form of hemoglobin - a
protein in red blood cells that carries oxygen
*the cells are not round, but sickle shaped…
*these cells rupture easily, cannot carry O2 well, and can clog
blood vessels
*found almost exclusively in African Americans
*heterozygous carriers may be less susceptible to malaria
- arose from people who live in Africa where malaria is most
common
-if are hetero, the few sickle cells that may be produced get
destroyed along with the malaria parasite!
*is recessive
Let's do a potential pedigree:
Hemophilia:
- blood does not clot quickly
- people who have this can bleed to death in a very short time
from a small wound if left untreated
- 12 genes code for blood clotting proteins, any mutation of these
can cause hemophilia
- 2 of these are found only on the X chromosome - this is a
sex-linked trait!
* what his means for males… if one gene is defective, no
other
X chromosome is there to compensate - he will have
hemophilia
Let's do a potential pedigree:
Color blindness is also sex-linked!
Also there are sex-influenced and sex-limited traits...
~ sex-influenced - if a male has one recessive allele, he will
show that trait, but it will take two recessive for the
female to show that same trait.
ex: baldness (paraphrase this!!)
In men the gene is dominant, while in women it is recessive. A man needs
only one allele (B) for the baldness trait to be expressed, while a bald
woman must be homozygous for the trait (BB). If B is the allele for
baldness and b is the allele for normal hair, a bald man can be heterozygous
(Bb) or homozygous bald (BB). A man with normal hair must be
homozygous normal (bb). A normal woman can be homozygous normal
(bb) or heterozygous (Bb). A woman who has thinning hair and a receding
hair line in later life must be homozygous bald (BB).
Sex
Baldness
Normal Hair
Male
BB, Bb
bb
Female
BB
Bb, bb
If you are a man with a bald father, you are doomed to lose your hair if
your father is homozygous bald (BB). If he is heterozygous (Bb), you have
a 50-50 chance of inheriting his gene for normal hair (b). If you also inherit
the gene for normal hair from your mother, then your genotype will be bb
and your phenotype will be normal hair:
Sperm
Gametes
B
b
b
Bb
bb
b
Bb
bb
Eggs
If your grandfather on the mother's side of your family is bald, then you
have a 50-50 chance of inheriting this gene from your mother:
Sperm
Gametes
b
B
b
Bb
Bb
bb
bb
Eggs
b
If your grandfather on the mother's side of your family is bald and your
father is heterozygous bald, then you have a 75% chance of losing your
hair:
Sperm
Gametes
B
B
b
Bb
Bb
Bb
bb
Eggs
b
If your grandfather on the mother's side of your family is bald and your
father is homozygous bald, then you have a 100% chance of losing your
hair:
Sperm
Gametes
B
B
B
BB
BB
Bb
Bb
Eggs
b
~ sex-limited - traits that are visible only within one sex
ex: ovary development in females, sperm in males
Phenylketonuria: (PKU)- individual doesn’t have the enzyme to
convert the AA phenylalanine to the AA tyrosine
* is autosomal recessive
* if left untreated, phenylalanine builds up in the body
causing
severe mental retardation
*can find warnings on pop cans and other foods high in
phenylalanine