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Chapter 78 -- Part 1: Law of Segregation
According to the law of segregation, the two traits carried by each parent separate during the formation of gametes.
Segregation and independent assortment of chromosomes result in genetic variation.
The mono hybrid cross is the best example of this law.
The F 1 generation of the mono hybrid cross has a trait that was not seen in either parent.
A cross between two organisms is called a mono hybrid cross.
The appearance ratio from this cross is 3 tall to 1 dwarf plant.
The ratio of the type of genes is 1:2.
The results are the same for any mono hybrid cross.
The testcross is a way to determine the dominant trait of an individual plant or animal.
There is a cross between real organisms.
A person is crossed with another person.
All offspring of the testcross will show the dominant trait if the individual being tested is in fact Homozygous dominant.
There can't be offspring with the same trait.
One-half of the offspring can be expected to show a trait if the individual is hybrid.
If any offspring show a trait that's different from the norm, the parent must be a hybrid.
There can't be white offspring if the parent isBB.
There is a 50% chance that any offspring will be white if the parent is a hybrid.
The law of independent assortment applies when a cross is carried out between two individuals hybrid for two or more different things.
This type of cross is called a di hybrid cross.
During gamete formation, the alleles of a gene for one trait, such as height, are separated from the alleles of a gene for another trait, such as seed color.
The genes for height and seed color are not on the same chromosomes.
The only factor that affects how these alleles are divided is how the pairs line up in metaphase of meiosis I, which is random.
If the genes for tall and short are linked to the genes for yellow and green seed color, the genes will not assort independently.
A plant with yellow seeds is tall.
Green seeds can be found if a plant is short.
The genes that are close to each other on the same chromosomes tend to move as a unit.
There is a cross that complies with the law of independent assortment.
Two flowers are cross-bred to have the same genes for height and seed color on different chromosomes.
The TY genes can only be produced by the YY parent.
The tt yy parent can only produce gametes with certain genes.
There is only one outcome that can be achieved in the F 1 and that is Tt Yy.
The F 1 generation has tall plants with yellow seeds.
They are known as di hybrid.
A cross between two F 1 plants is called a di hybrid cross because it is a cross between individuals that are hybrid for two different characteristics.
Four different types of gametes can be produced by this cross.
The Punnett square is shown in the figure below.
All the ratios are out of 16 because there are 16 squares.
You don't need to pay attention to the many different genotypes that are possible in the F 2 generation.
Pay attention to the ratio of the di hybrid cross.
It is 9 tall, yellow, 3 tall, green, and 3 short, yellow.
There are only two alleles for a trait that is determined by a single gene.
Pea plants have only two alleles, one of which is tall.
We will consider situations in which two or more genes are involved.
Simple inheritance cannot explain the inheritance pattern.
Blending is a sign of incomplete dominance.
There are two examples.
A long watermelon is crossed with a round watermelon and produces all the watermelons.
All gray animals are produced when a black animal is crossed with a white animal.
The convention for writing genes is capital letters since neither trait is dominant.
There is a 25 percent chance that the offspring will be red, a 25 percent chance that the offspring will be white, and a 50 percent chance that the offspring will be pink.
A blend is incomplete dominance.
Both qualities show in codominance.
Both traits show in codominance.
There are blood groups in humans.
There are three different blood groups.
The groups are based on the surface of the red blood cells.
There are two allelic variant that can be found at a single gene locus.
A person can have both MM and NN on their red blood cells, or they can be hybrid and have both MM and NN on their red blood cells.
The M and N phenotypes are not intermediate.
Both M and N are expressed by the surface of the red blood cells.
Pea plants can be either tall or short.
Multiple alleles are when there are more than two allelic forms of a gene.
There are four different blood types in humans and they are determined by the presence of specific molecule on the surface of the red blood cells.
The four different blood types are determined by the three alleles A, B, and O. IA and IB are often written as A and B.
The person has anAB blood type when both alleles are present.
O is a trait that is often written as i.
A person can have any of the six blood types.
Multiple genes are the source of many traits.
Many characteristics such as skin color, hair color, and height are a result of a blend of several different genes.
These are known as polygenic.
Two parents have more genes for short than for tall.
They can have a child with genes that make them taller than their parents.
The distribution of skin pigmentation is shown.
Extranuclear genes and genomic imprinting are exceptions to inheritance.
If a trait is passed down from the mother to the father, it's called genetic imprinting.
It occurs during gamete formation and is caused by the silencing of a particular allele.
Only one of the imprinted genes is expressed by a zygote.
The imprint is passed down from generation to generation.
Sex-linked genes are located on the X chromosomes, but imprinted genes are located on autosomes.
Extranuclear genes are located in the nucleus of the cell.
A small number of genes are carried by the small circular DNA in these organelles.
There are several rare and severe inherited diseases in humans.
Weak muscles are caused by defects in the genes involved in energy production.
The father's mitochondria do not enter the egg during fertilization, so the mother's genes are the only ones she has.
The environment can change the expression of genes.
The temperature of the environment can affect the expression of the vestigial wings in fruit flies.
When raised in a hot environment, fruit flies can grow wings almost as long as normal wild-type wings.
Many human diseases are multifactorial.
There is an underlying genetic component.
Some examples are heart disease, diabetes, cancer, alcoholism, and schizophrenia.
Intelligence is the result of an interaction of genetic predisposition and the environment.
The expression of the geno in an organisms is influenced by environmental factors.
Penetrance is the proportion or percentage of individuals in a group with the same genetics.
Many people who inherit a genetic variation of the breast cancer risk factor develop the disease in their lifetimes.
Some people with the allele don't get breast cancer.
Maybe this is caused by a factor from the environment.
The genes are called linked genes.
Thousands of genes are linked because there are many more genes than chromosomes.
Every cell has 46 chromosomes.
Humans have 46 linkage groups.
Unless they are separated by a crossover event during meiosis and gamete formation, linked genes tend to be inherited together.
Sex-linked traits can be found on the X chromosomes.
Sex chromosomes X and Y are found on 44 of the 46 human chromosomes.
There are few genes on the Y.
There are two copies of the sex-linked genes in females.
A female will only express her sex-linked trait if she carries two different genes.
She will be a carrier if she only carries one X-linked gene.
If a sex-linked trait is due to a dominant mutation, a female will only express the phenotype with a single variant of the X-X gene.
Males only have one X-linked gene.
The male will express the X-Y gene if he inherit it.
Sex-linked conditions are more common in males than in females.
Sex-linked traits are not the only ones where expression depends on the sex of the individual.
Human males can't produce milk because of their mammary glands.
There are some important facts about sex.
Some examples of sex-linked diseases are color blindness, and Duchenne muscular dystrophy.
The daughters of affected fathers are carriers.
The son cannot inherit a sex-linked trait from his father.
A boy has a 50 percent chance of inheriting a sex-linked trait from his mother.
There is no carrier state for X-linked traits in males.
The male will express the genes if he has them.
It is rare for a female to have a sex-linked condition.
She needs to inherit a gene from both of her parents in order to be affected.
The farther apart the two genes are, the more likely they will be separated during meiosis.
A physical bridge built around the point of exchange can be seen at the site of a crossover and recombination.
A cross-over is a recombination.
Heterogeneity is a major source of variation in sexually reproducing organisms.
A function of the distance between genes is the probability that genes on the same chromosomes will separate.
The resulting four gametes would contain the following genes.
There are two different types of gametes.
Four gametes contain the following genes: Ab, aB, and Ab.
There are different types of gametes.
Recombination occurs 1 percent of the time if the map unit distance on a chromosomes is one.
The order of the linked genes on the chromosomes is shown by the rate of crossover.
This is an example.
Three genes are linked.
The frequencies for B and D are 5 percent, for B and A are 30 percent, and for D and A is 25 percent.
The data can be used to create a linkage map.
The results of the experiment do not match the predictions.
The genes for wing size and body color are on the same chromosomes.
The existence of small numbers of nonparental phenotypes can only be explained by occasional breaks in the linkage.
It's important to remember that crossing-over accounts for the recombination of genes.
This is how to determine the recombination Frequency.
You can use the results from the cross.
The Gg and Nn genes are linked.
A family tree is a representation of a trait being studied for every member of the family.
Genetics use the pedigree to determine how a trait is passed down.
Females are represented by a circle and males by a square.
Sometimes the carrier state is not shown.
Sometimes it is represented by a half-shaded-in shape.
A shape is shaded if a person has that trait.
Determine the pattern of inheritance.
Eliminate all possibilities first.
In order for a child to have dominance, she or he would have had to receive a single mutant gene from one afflicted parent, and that's not the case.
In order for F 3 generation daughter #1 to have the condition, she would have had to inherit two genes from each parent.
Her father does not have the condition.
The trait must be related to the person.
One of the X chromosomes in the embryo of a female mammal is inactivated.
This happens randomly.
Some cells have one X inactivated, some cells have the other X inactivated in an embryo that is a genetic mosaic.
The cells of female mammals are not the same.
The nucleus of the female's cells can be seen at the outer edge of a dark spot on the inactivated chromosomes.
The Barr body is a dark spot.
Female body cells have one Barr body.
Normal male cells do not have any.
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