Edited Invalid date
Chapter 8 -- Part 1: Heredity
The material in this chapter will help you solve genetics problems.
A genetics problem is an analysis of the characteristics of parents and offspring.
You have to determine the traits of the other generation based on the traits of one generation.
The application of probability rules is required for genetics problems.
There is a chance that the coin will be heads.
If a coin is thrown again, there is a chance that it will be heads.
The first toss does not affect the second toss.
You simply add the probabilities of each event happening separately to determine the probability of two or more independent events.
The probability of getting two heads is 1/2 x 1/2.
The probability of three heads is 1/2 x 1/2 x 1/2.
The formula for carrying out these instructions is described by a genetic code.
Pea plants have a gene that codes for stem length.
There are two alleles of the gene for stem length in pea plants, one for tall plants and one for dwarf plants.
Every gene has a unique location.
One of the chromosomes was contributed by each parent.
The stem length on one pea plant's chromosomes might be different from the stem length on the second parent's chromosomes.
The actual trait is expressed by the dominant allele.
The tall stem allele is dominant in pea plants.
If a pea plant has one of the alleles, the dominant one, it will produce tall plants.
A dominant allele is represented by a capital letter, while a recessive allele is represented by a lower form of the same letter.
The first letter of the dominant allele is often used to represent the gene.
The dominant trait is expressed.
It is normal to write a pair of alleles with the dominant one first.
Only the dominant allele is expressed in this condition.
The tall stems, blue eyes, and brown hair all have the same allele.
The term geno is used to describe all of an individual's alleles, and its phenotype is used to describe the expression of those alleles.
The laws of segregation and independent assortment were discovered by a nineteenth-century monk.
The laws describe the separation of chromosomes.
The rules of probability can be used to describe how the different chromosomes are assembled in offspring.
Each gamete contains only one copy of each of the two chromosomes, so that one member of each pair migrates to the opposite pole.
The process is not influenced by the migration of genes within one pair of chromosomes to opposite poles.
A cross between a tall pea plant and a dwarf pea plant is a mono hybrid cross because it is investigating a gene for only one trait.
A cross looking at the stem length and flower color is a di hybrid cross.
A cross investigating seed color and texture is another example.
The first thing to do in analyzing the genetics problem is to figure out which parents produced which gametes.
The law of segregation states that each chromosome migrates to an opposite pole and ends up in a separate gamete.
The next step is to figure out how the gametes can combine.
The easiest way to accomplish this is to create a Punnett square.
There are four boxes in the middle that combine the allele found at the top with the one found to the left.
The possibilities of combining the two gametes from one parent with the two gametes from the second parent are represented by the four boxes.
The genotypic frequencies of the offspring are represented by these results.
The plants are 1/2 tall and 1/2 dwarf.
Results can be given as frequencies, percents, or ratios.
The dwarf to tall plants ratio is 2:1.
The alleles of all possible gametes are the first step in analyzing this cross.
The law of segregation says that one allele of each pair migrates to opposite poles and ends up in separate gametes.
The next step is to figure out how the gametes from one and the other parent can be combined.
In the 16 boxes of the square, the gametes at the top and left are combined.
The number of times each genotype appears is the next step.
The final step is to count how many times each phenotype appears.
Some of the phenotypes have more than one genotype.
There are nine plants with yellow and round seeds, three plants with green and round seeds, three plants with yellow and wrinkled seeds, and one plant with green and wrinkled seeds in the F2 progeny.
The same ratio was observed in his experiments for this cross.
You want to know the genetics of a dwarf pea plant.
You want to know the genetics of a plant with tall stems.
A test cross is used to determine which genotype is correct.
You will always be aware of the individual's genetic makeup.
If you don't know the second allele for the first individual, leave a blank space for it with an underscore.
There is an analogous situation presented by a coin toss.
Half the time it will be tails if you toss a coin six times.
There is a small chance that all six tosses will be tails.
There are times when the alleles for a gene don't exhibit the dominant and recessive behaviors.
Sometimes both alleles are written with the same letter but with a different number or letter to differentiate them.
There may be no apparent rationale for the method used to indicate different alleles.
Both inherited alleles are expressed in this pattern.
The M and N blood types produce substances on the surface of human red blood cells.
Imagine a continuum to help you distinguish inheritance.
There is complete dominance by a dominant allele.
Both alleles are expressed at the other extreme.
A blend of two different alleles produces an intermediate phenotype.
The A, B,AB, and O types correspond to the presence or absence of an A or B sugar component in red blood cells.
Half of the A sugar is attached and the other half is attached to the B sugar.
Blood transfusions must be made between people with similar characteristics.
O type blood contains no A or B sugars, so anyone can accept it.
A person with O type blood is a universal donor for the ABO blood group.
Yellow and green pea seeds and A, B, and O blood types are examples of a range of varieties.
The law of independent assortment states that if two genes are on different chromosomes, they should be separated.
The law of independent assortment states that genes that are linked do not obey it.
The body color and wing structure are affected by two of these genes.
About 18% of the time, linked genes cross over during prophase I.
The more places between the genes that the chromosomes can break, the more likely the two genes will cross over during synapsis.
Recombination frequencies are used to give a picture of the arrangement of genes.
Animals have one pair of chromosomes that do not have the same genes.
In mammals and fruit flies, there are two X chromosomes that make up a female and one X and one Y that make up a male.
The Y chromosomes have relatively few genes and are small compared to the X.
There are additional considerations when working with sex linked genes.
Two copies of the sex-linked gene are given to females when they inherit a sex-linked genes.
This is the same situation as for autosomal inheritance.
A male will only inherit one copy of the gene if he has the X chromosome.
The Y chromosome does not deliver a similar gene.
The X chromosome of a male is the only one that expresses his trait regardless of whether it is dominant or not.
Color blindness, as well as other sex-linked genetic defects, are more common in males.
Female mammals have two X chromosomes in each cell, but one of them does not leave the nucleus of the cell.
Most of the genes are not expressed nor do they interact with the X chromosomes that are expressed.
Only the genes on the one active X chromosome are expressed by that cell.
One of the two chromosomes in each embryo becomes inactive when X-inactivation begins.
The X chromosomes from the progenitor cell will be inactivated as will the X chromosomes from the subsequent daughter cells.
Some groups of cells will have one X chromosomes inactivated, while other groups will have the other X chromosomes.
All of the cells in a mammal are not the same.
A very visible example of X-inactivation can be seen in the different groups of cells that produce different patches of color.
The cats have hair that is randomly arranged over their bodies.
The orange and black colors are determined by a gene on the X chromosomes.
The hair is black when the X chromosome with the orange allele is inactivated.
There are patches with the black allele that are orange.
Gametes with extra or missing chromosomes can be produced by the failure to separate two or more chromosomes.
During anaphase, the failure of two chromatids of a single chromosomes causes daughter cells with extra or missing chromosomes.
If a polyploid gamete is fertilized with a similar gamete, a polyploidy zygote can be formed.
Plants have polyploidy.
Gene function can be affected by most point mutations.
When lowoxygen conditions occur, the red blood cell becomes sickle shaped because of the defects in the hemoglobin molecule.
Red blood cells don't flow through the capillaries freely and oxygen isn't delivered throughout the body.
Brain cells die when these fats accumulate in the nerve cells of the brain.
Nondisjunction is the most common cause.
Most aneuploid gametes do not produce viable offspring, but some do.
These can lead to genetic disorders.
Down syndrome individuals have many defects, including mental retardation, heart defects, respiratory problems, and deformities in external features.
There are either sex chromosomes or no sex chromosomes in the embryo.
sterile females with physical abnormality are the ones who have eggs.
The absence of the Y chromosome, with its few malenecessary genes, is not nearly as harmful as the absence of a single chromosomes.
View flashcards and assignments made for the note
Getting your flashcards
Privacy & Terms