But just what do these terms mean? Before defining homozygous and heterozygous, we have to first think about genes. Each of your cells contains very long stretches of DNA deoxyribonucleic acid. This is heritable material that you get from each of your parents. DNA is composed of a series of individual components called nucleotides. There are four different types of nucleotides in DNA:.
Inside the cell, DNA is usually found bundled up into chromosomes found in 23 different pairs. Genes are very specific segments of DNA with a distinct purpose. These segments are used by other machinery inside the cell to make specific proteins. Proteins are the building blocks used in many critical roles inside the body, including structural support, cell signaling, chemical reaction facilitation, and transport. The cell makes protein out of its building blocks, amino acids by reading the sequence of nucleotides found in the DNA.
The cell uses a sort of translation system to use information in the DNA to build specific proteins with specific structures and functions. Specific genes in the body fulfill distinct roles.
For example, hemoglobin is a complex protein molecule that works to carry oxygen in the blood. Several different genes found in the DNA are used by the cell to make the specific protein shapes needed for this purpose. You inherit DNA from your parents. Broadly speaking, half of your DNA comes from your mother and the other half from your father. For most genes, you inherit one copy from your mother and one from your father.
However, there is an exception involving a specific pair of chromosomes called sex chromosomes. Because of the way sex chromosomes work, males only inherit a single copy of certain genes.
The genetic code of human beings is quite similar: Well over 99 percent of nucleotides that are part of genes are the same across all humans.
However, there are some variations in the sequence of nucleotides in specific genes. These different variations of genes are called alleles. They might cause a small difference in the protein that makes it work slightly differently. A person is said to be homozygous for a gene if they have two identical copies of the gene.
Heterozygous just means that a person has two different versions of the gene one inherited from one parent, and the other from the other parent.
Homozygous: You inherit the same version of the gene from each parent, so you have two matching genes. Heterozygous: You inherit a different version of a gene from each parent. They do not match. However, other specific mutations can lead to human disease. One example is sickle cell anemia. This causes an important change in the configuration of hemoglobin. Because of this, red blood cells carrying hemoglobin begin to break down prematurely.
This can lead to problems like anemia and shortness of breath. Generally speaking, there are three different possibilities:. People who are heterozygous for the sickle cell gene have one unaffected copy of the gene from one parent and one affected copy of the gene from the other parent. Heterozygotes can get genetic disease, but it depends on the type of disease.
In some types of genetic diseases , a heterozygous individual is almost certain to get the disease. In diseases caused by what are called dominant genes, a person needs only one bad copy of a gene to have problems. Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square. Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares. This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.
These will be the odds every time a new offspring is conceived by parents with YG genotypes. An offspring's genotype is the result of the combination of genes in the sex cells or gametes sperm and ova that came together in its conception.
One sex cell came from each parent. Sex cells normally only have one copy of the gene for each trait e. Each of the two Punnett square boxes in which the parent genes for a trait are placed across the top or on the left side actually represents one of the two possible genotypes for a parent sex cell. Which of the two parental copies of a gene is inherited depends on which sex cell is inherited--it is a matter of chance.
If you are not yet clear about how to make a Punnett Square and interpret its result, take the time to try to figure it out before going on.
Why is it important for you to know about Punnett squares? The answer is that they can be used as predictive tools when considering having children. Let us assume, for instance, that both you and your mate are carriers for a particularly unpleasant genetically inherited disease such as cystic fibrosis. Of course, you are worried about whether your children will be healthy and normal. For this example, let us define "A" as being the dominant normal allele and "a" as the recessive abnormal one that is responsible for cystic fibrosis.
As carriers, you and your mate are both heterozygous Aa. From these genotypes, we find a phenotypic ratio of 9 round—yellow:3 round—green:3 wrinkled—yellow:1 wrinkled—green. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size. The physical basis for the law of independent assortment also lies in meiosis I, in which the different homologous pairs line up in random orientations.
Each gamete can contain any combination of paternal and maternal chromosomes and therefore the genes on them because the orientation of tetrads on the metaphase plane is random Figure 8. Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur.
It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur.
Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur. An example of a genetic event is a round seed produced by a pea plant. When the F 1 plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was now three out of four.
In other words, in a large population of F 2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds. Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses. Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring. As will be discussed, Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses.
For instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds still produced offspring that had a ratio of green:yellow seeds ignoring seed texture and a ratio of round:wrinkled seeds ignoring seed color. The characteristics of color and texture did not influence each other.
The product rule of probability can be applied to this phenomenon of the independent transmission of characteristics. The product rule states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone.
To demonstrate the product rule, imagine that you are rolling a six-sided die D and flipping a penny P at the same time. The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this action, and each event is expected to occur with equal probability.
For example, consider how the product rule is applied to the dihybrid cross: the probability of having both dominant traits in the F 2 progeny is the product of the probabilities of having the dominant trait for each characteristic, as shown here:. On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes that can come about by more than one pathway. The sum rule states that the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities.
What is the probability of one coin coming up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads P H and the quarter may be tails Q T , or the quarter may be heads Q H and the penny may be tails P T. Either case fulfills the outcome. You should also notice that we used the product rule to calculate the probability of P H and Q T , and also the probability of P T and Q H , before we summed them.
Again, the sum rule can be applied to show the probability of having just one dominant trait in the F 2 generation of a dihybrid cross:. To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him calculate the probabilities of the traits appearing in his F 2 generation.
Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications.
In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype.
When true-breeding, or homozygous, individuals that differ for a certain trait are crossed, all of the offspring will be heterozygous for that trait. If the traits are inherited as dominant and recessive, the F 1 offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring are self-crossed, the resulting F 2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive.
Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F 2 offspring will exhibit a ratio of three dominant to one recessive. Mendel postulated that genes characteristics are inherited as pairs of alleles traits that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual.
In addition, genes are assorted into gametes independently of one another. That is, in general, alleles are not more likely to segregate into a gamete with a particular allele of another gene. Punnett square: a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid.
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