Chapter 4 • Lesson 22
Genetics
Objective:;
Key Words
• alleles • dominant • recessive • homozygous • heterozygous • genotype • phenotype true-breeding • law of segregation • law of independent assortment • incomplete dominance codominance • multiple alleles • polygenic trait • sex-linked trait • karyotype
Getting the Idea
In the mid-1800s, the Austrian monk Gregor Mendel made a discovery that paved the way for our current understanding of genetics. Using pea plants, Mendel studied how traits are passed from one generation to the next. Although his work was largely ignored until 1900, his ideas eventually became the foundation for the basic principles of heredity.
Dominant and Recessive Traits
Many organisms, including humans, reproduce sexually. Organisms that reproduce sexually receive genes from both their parents. In general, both parents contribute genes for the same traits. Those genes may be the same, or they may code for different forms of a trait, such as tall and short height. The different forms of the gene for a specific trait are called alleles.
An organism that receives two identical alleles for a characteristic shows that characteristic. However, when an organism has two different alleles for a trait, the dominant allele is expressed. The organism does not have the characteristic specified by the recessive allele. Recessive alleles are expressed only when no dominant alleles are present. The table lists some dominant and recessive traits in humans.
Recessive
Absent Straight Attached Absent
Dominant alleles are represented by uppercase letters, and recessive alleles are represented by lowercase letters. For example, black fur is dominant over brown fur in a certain type of rabbit. A rabbit's genes could be described as 66 (two dominant alleles), Bb (one dominant and one recessive allele), or bb (two recessive alleles). Remember that the recessive trait, brown fur, appears only when no dominant alleles are present. So a rabbit will have black fur if its alleles are 66 or Bb, and it will have brown fur only if its alleles are bb.
An organism that has two dominant or two recessive alleles for a trait is homozygous for that trait. An organism is heterozygous if it has two different alleles for a trait.
Genotype and Phenotype
The complete set of genes carried by an organism make up its genotype. A genotype includes the alleles that are not expressed as well as those that are. The genotype of a rabbit with black fur may include the alleles for both black and brown fur. An organism's phenotype is the set of traits that the organism displays. For the rabbit described above, black fur is part of its phenotype. Its phenotype does not include brown fur.
Mendelian Genetics
For his experiments on inherited traits, Mendel used pea plants because they can self-pollinate to produce true-breeding offspring. These offspring are genetically identical to each other and to their parent and are homozygous for their traits. Once he had produced true-breeding strains, Mendel began to interbreed the strains. In one experiment, Mendel crossed true-breeding green-pod plants with true-breeding yellow-pod plants. The results are shown below.
All the offspring had green pods. How did this happen? Pea pods can be either green or yellow. However, because the parent plants (parent, or P, generation) were true-breeding, they were homozygous. The green-pod pea plants had two alleles for green pods (GG), and the yellow-pod pea plants had two alleles for yellow pods (gg). When they were interbred, each parent contributed one allele to the offspring (first, or F^ generation). The cross produced heterozygous offspring, each of which had one allele for green pods and one allele for yellow pods (Gg).
Mendel developed the law of segregation to describe how an organism's alleles are transferred to its gametes, or sex cells. The law of segregation states that when an organism produces gametes, its two alleles for a trait separate and go into different gametes. In this way, when gametes from two parents join to produce a new organism, the offspring receives one allele from each parent.
Does the gene for one trait have anything to do with whether the gene for another trait is inherited? Are they linked? Mendel showed that different traits are inherited separately. This is called the law of independent assortment. You will learn more about independent assortment in the next lesson.
Punnett Squares
Scientists can use the laws of probability to predict the outcomes of some genetic crosses. A diagram called a Punnett square is useful for finding the probable results of a simple genetic cross.
A Punnett square is made by dividing a square into sections. Letters representing the alleles of one parent are written across the top of the square. Letters representing the alleles of the other parent are written down the side. To complete the square, the alleles of one parent are combined with those of the other in each box.
Recall that Mendel crossed pea plants that were homozygous for green pods (GG) with pea plants that were homozygous for yellow pods (gg). The Punnett square below shows the results of this cross.
A Punnett square describes probability, or the mathematical chance of an event. According to the Punnett square shown above, the offspring can have only one genotype—Gg. The genotypic ratio is 4:0 (100% Gg). The only possible phenotype of the offspring is green, producing a phenotypic ratio of 4:0 (100% green). What would the genotypic and phenotypic ratios be if two of the offspring from the ft generation were crossed? The Punnett square below shows this outcome.
In this cross, both parents are heterozygous for green pods. Each has one allele for each pod color. The result in the second, or F2, generation is a variety of offspring. The possible genotypes are one GG, two Gg, and one gg. This yields a genotypic ratio of 1:2:1 (25% GG, 50% Gg, 25% gg). The possible phenotypes are green (in plants with one or two dominant alleles) and yellow (in plants with two recessive alleles). So, the phenotypic ratio is 3:1 (75% green, 25% yellow).
Intermediate Traits
Some traits cannot be explained by simple dominant or recessive alleles. In fact, most genes have more than two alleles, and many traits are controlled by more than one gene. For some traits, neither allele is dominant over the other. In this situation, called incomplete dominance, the result is a blend of the two forms of the trait. Snapdragon plants are an example of incomplete dominance. A cross between a plant with red flowers and a plant with white flowers produces offspring with pink flowers.
Alleles for some other traits exhibit codominance, a condition in which both alleles are expressed in the same organism. In some types of chickens, the alleles for feather color exhibit codominance. Mendel would have expected the heterozygous offspring of a black-feathered rooster and a white-feathered hen to be either black or white. However, the offspring actually have some black feathers and some white feathers. In codominance, both alleles are expressed equally. This is different from incomplete dominance, which results in a phenotype that is a blend of the two.
Some traits are determined by multiple alleles. Although each organism has only two alleles for the trait, more than two possible alleles exist in the population. Human blood types are an example of such a trait. The possible phenotypes for human blood type are A, B, AB, and O. Although each person can have at most two alleles for blood type, there are three alleles for blood type in the human population, A, B, and O. Two of these alleles, A and B, are codominant.
The alleles for human blood type are written as IA, IB, and /. It is important to remember that IA and IB are codominant, but both are dominant over /. The genotypes IA IA and lAi produce type A blood. The genotypes IBIB and lBi produce type B blood. People with the genotype IA IB have type AB blood, and the genotype //' codes for type O blood. The Punnett square below illustrates how two heterozygous parents, one type A and one type B, can produce offspring with any blood type.
Because of the way blood type is inherited, it is often possible to identify whether an individual is the parent of a child based on that child's blood type. Suppose a man with type AB blood marries a woman with type B blood. At the time of the marriage, the woman already has two children. The couple then have two children together. Of the four children, one child has type AB blood, one has type B blood, one has type A blood, and one has type O blood
Because the man has type AB blood, you know his genotype is IAIB. The woman's genotype could be either IBIB or I8i, but she has one child with type O blood. Type O results from the inheritance of two recessive alleles, so you know that the woman is heterozygous for type B blood. Her genotype must be \B\. You can use this information and a Punnett square to determine the possible genotypes of the offspring the man and woman can produce.
Using the Punnett square, you can determine that the man is not the father of the child with type O blood. However, he could be the father of the children with type AB, type A, or type B blood.
Recall that some traits are controlled by more than one gene. A polygenic trait is a trait that is controlled by two or more genes. Height in humans is an example of a polygenic trait. Because it is controlled by multiple genes, humans can have a large range of heights, rather than all having a few exact heights. Other examples of polygenic traits in humans are eye color and skin color. At least three genes interact to produce a wide range of eye colors in humans. More than four genes control skin color in humans, leading to a wide range of variation in this trait.
The table below summarizes the inheritance of intermediate traits.
Traits Affected by Sex Chromosomes
Humans and other mammals have two types of chromosomes: autosomes and sex chromosomes. An autosome is any chromosome other than a sex chromosome. The sex chromosomes are the X and Y chromosomes, which determine gender. When an offspring receives a set of chromosomes from each parent, the set includes one sex chromosome from each parent. In mammals, females have two X chromosomes. Therefore, a female can contribute only an X chromosome to her offspring. The male sex chromosomes are XY, so a male can contribute either an X or a Y chromosome to his offspring. In humans and other mammals, the male determines the gender of the offspring. He has an equal chance of passing down either an X or a Y sex chromosome.
Some traits other than gender are sex-linked traits, meaning they are determined by alleles located on the sex chromosomes. Most sex-linked traits are recessive and carried on the X chromosome. Because males have only one X chromosome, a male who carries the recessive allele will show the sex-linked recessive trait. Females have two X chromosomes, so a female must inherit two recessive alleles for the recessive trait to be expressed. Albinism, hemophilia, color blindness, and muscular dystrophy are sex-linked genetic disorders. You will read more about genetic disorders in Lesson 25.
Karyotypes
Scientists sometimes study entire chromosomes rather than individual genes to learn more about the traits an organism inherits. Karyotypes are one of the tools used to conduct such studies. A karyotype is a visual display of an individual's chromosomes, which are paired and numbered.
Karyotypes clearly show an individual's gender. Look at the last pair of chromosomes in the karyotype below. The diagram clearly shows the different lengths of the X and Y chromosomes, which determine gender. Karyotypes can also indicate chromosomal abnormalities that result from broken, missing, or extra chromosomes. Some of these abnormalities can produce miscarriages, birth defects, or developmental problems in offspring. The karyotype below shows the chromosomes of a person with Down syndrome. Down syndrome is a genetic condition in which an individual receives three copies of chromosome 21 instead of two.
Focus on Inquiry
A Punnett square is a model. Like all scientific models, Punnett squares are based on scientific information. They can be tested by experiments.
The Punnett square below shows a cross between a four o'clock plant with red flowers and one with white flowers. In these plants, neither allele for flower color is dominant over the other. Therefore, capital and lowercase letters are not used to represent traits. Instead, a letter, in this case C for color, is used with a superscript, in this case R for red and W for white. The genotype for a plant with red flowers is written as CRCR, and the genotype for a plant with white flowers is written CWC^. A hybrid offspring is written CRCW. This is shown in the partially completed Punnett square below.
Complete the Punnett square to show the genotypes of the remaining two offspring. What phenotypes do you think these offspring would show?
What could you learn by actually crossing two plants that you cannot find out from the Punnett square?
Fill in the blank Punnett square below to find the genotypes of the plants in the F2 generation.
What is the phenotypic ratio of these offspring?
What offspring would you expect if you crossed an F1 hybrid and a plant with red flowers?