Figure 6.4
Pink snapdragons are an example of incomplete dominance.
Another example of incomplete dominance is sickle cell anemia, a disease in which the hemoglobin protein is produced incorrectly and the red blood cells have a sickle shape. A person that is homozygous recessive for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant will have normal red blood cells. And because this trait has an incomplete dominance pattern of expression, a person who is heterozygous for the sickle cell trait will have some misshapen cells and some normal cells (Figures below and below). These heterozygous individuals have a fitness advantage; they are resistant to severe malaria. Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits.
Figure 6.5
Sickle cell anemia causes red blood cells to become misshapen and curved (upper figure) unlike normal, rounded red blood cells (lower figure).
Figure 6.6
Sickle cell anemia causes red blood cells to become misshapen and curved (upper figure) unlike normal, rounded red blood cells (lower figure).
An example of a codominant trait is ABO blood types (Figure below), named for the carbohydrate attachment on the outside of the blood cell. In this case, two alleles are dominant and completely expressed (designated IA and IB), while one allele is recessive (i). The IA allele encodes for red blood cells with the A antigen, while the IB allele encodes for red blood cells with the B antigen. The recessive allele (i) doesn’t encode for any antigens. An antigen is a substance that provokes an immune response, your body’s defenses against disease, which will be discussed further in the Diseases and the Body's Defenses chapter. Therefore a person with two recessive alleles (ii) has type O blood. As no dominant (IA and IB) allele is present, the person cannot have type A or type B blood.
There are two possible genotypes for type A blood, homozygous (IAIA) and heterozygous (IAi), and two possible genotypes for type B blood (IBi and IBIB). If a person is heterozygous for both the IA and IB alleles, they will express both and have type AB blood with both antigens on each red blood cell. This pattern of inheritance is significantly different than Mendel’s rules for inheritance because both alleles are expressed completely and one does not mask the other.
Figure 6.7
An example of codominant inheritance is ABO blood types.
Polygenic Traits and Environmental Influences
Another exception to Mendel’s rules is polygenic inheritance, which is when a trait is controlled by more than one gene. Often these traits are in fact controlled by many genes on many chromosomes. Each dominant allele has an additive effect, so the resulting offspring can have a variety of genotypes, from no dominant alleles to several dominant alleles. In humans, some examples of polygenic traits are height and skin color. People are neither short nor tall, as was seen with the pea plants studied by Mendel, which has only one gene that encodes for height. Instead, people have a range of heights determined by many genes. Similarly, people have a wide range of skin colors. Polygenic inheritance often results in a bell shaped curve when you analyze the population (Figure below). That means that most people are intermediate in the phenotype, such as average height, while very few people are at the extremes, such as very tall or very short.
Figure 6.8
Polygenic traits tend to result in a distribution that resembles a bell-shaped curve, with few at the extremes and most in the middle. There may be 4 or 6 or more alleles involved in the phenotype. At the left extreme, individuals are completely dominant for all alleles, and at the other extreme, individuals are completely recessive for all alleles. Individuals in the middle have various combinations of recessive and dominant alleles. Other polygenic traits in dairy cattle are of extreme economic importance in agriculture .
Most polygenetic traits are partially influenced by the environment. For example, height is partially influenced by nutrition in childhood. If a child is genetically programmed to be average height but does not get a proper diet, he or she may be below average in size.
Other examples of environmentally influenced traits are mental illnesses like schizophrenia and depression. A person may be genetically predisposed to have depression, so when that person's environment contributes major stresses like losing a job or losing a close relative, the person is more likely to become depressed.
Linkage
Linkage refers to particular genetic position or loci, of alleles inherited together, suggesting that they are physically on the same chromosome, and located close together on that chromosome. A crossing-over event during prophase I of meiosis is rare between linked loci. Alleles for genes on different chromosomes are not linked; they sort independently (independent assortment) of each other during meiosis.
A gene is also said to be linked to a chromosome if it is physically located on that chromosome. For example, a gene (or loci) is said to be linked to the X-chromosome if it is physically located on the X-chromosome.
Linkage Maps
The frequency of recombination refers to the rate of crossing-over (recombination) events between two loci. This frequency can be used to estimate genetic distances between the two loci, and create a linkage map. In other words, the frequency can be used to estimate how close or how far apart the two loci are on the chromosome.
In the early 20th century, Thomas Hunt Morgan demonstrated that the amount of crossing over between linked genes differs. This led to the idea that the frequency of crossover events would indicate the distance separating genes on a chromosome. Morgan's student, Alfred Sturtevant, developed the first genetic map, also called a linkage map.
Sturtevant proposed that the farther apart linked genes were on a chromosome, the greater the chance that non-sister chromatids would cross over in the region between the genes during meiosis. By determining the number of recombinants - offspring in which a cross-over event has occurred - it is possible to determine the approximate distance between the genes. This distance is called a genetic map unit (m.u.), or a centimorgan, and is defined as the distance between genes for which one product of meiosis in 100 products is a recombinant. So, a recombinant frequency of 1% (1 out of 100) is equivalent to 1 m.u. Loci with a recombinant frequency of 10% would be separated by 10 m.u. The recombination frequency will be 50% when two genes are widely separated on the same chromosome or are located on different chromosomes. This is the natural result of independent assortment. Linked genes have recombination frequencies less than 50%.
Determining recombination frequencies between genes located on the same chromosome allows a linkage map to be developed. Linkage mapping is critical for identifying the location of genes that cause genetic diseases.
Sequencing the genome of agriculturally important animals, such as cattle, can be important in the improvement of production. For more information on this aspect of modern genetics, please see http://www.physorg.com/news188148947.html
Lesson Summary
Variants of genes are called alleles.
Genotype is the combination of alleles that an individual has for a certain gene, while phenotype is the appearance caused by the expression of the genotype.
Incomplete dominance and codominance do not fit Mendel’s rules because one allele does not entirely mask the other.
In polygenic inheritance, many genes control a trait with each dominant allele having an additive effect.
Review Questions
What is a variant of a gene that occurs at the same place on homologous chromosomes?
What is the type of allele that only affects the phenotype in the homozygous condition?
What type of allele masks the expression of the recessive allele and is therefore expressed in the heterozygote?
What is the term for the specific alleles of an individual for a particular trait?
What is the term for the appearance of the organism, as determined by the genotype?
If a organism has a certain phenotype, such as a tall pea plant, does that mean it must have the same genotype?
What is the term for the pattern of inheritance where an individual has an intermediate phenotype between the two parents?
IQ in humans varies in humans with most people having an IQ of around 100, and with a few people at the extremes, such as 50 or 150. What type of inheritance do you think this might describe?
A dark purple flower is crossed with a white flower of the same species and the offspring have light purple flowers. What type of inheritance does this describe?
What is the inheritance pattern where both alleles are expressed?
Further Reading / Supplemental Links
http://en.wikipedia.org/wiki/Dominant_gene
http://en.wikipedia.org/wiki/Polygenic_inheritance
http://staff.jccc.net/pdecell/evolution/polygen.html
http://www.curiosityrats.com/genetics.html
http://www.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html
Vocabulary
allele
An alternative form of a gene.
co-dominance
A pattern of inheritance where both alleles are equally expressed.
genotype
The genetic makeup of a cell or organism, defined by certain alleles for a particular trait.
homozygous
Having identical alleles for a particular trait.
heterozygous
Having two different alleles for a particular trait.
incomplete dominance
A pattern of inheritance where the offspring has a phenotype that is halfway between the two parents’ phenotypes.
phenotype
The physical appearance that is a result of the genotype.
polygenic inheritance
A pattern of inheritance where the trait is controlled by many genes and each dominant allele has an additive effect.
Points to Consider
Hypothesize about the genetic differences between males and females.
Can you name any human genetic disorders?
If a baby inherits an extra chromosome, what might the result be?
Lesson 6.3: Human Genetics
Lesson Objectives
List the two types of chromosomes in the human genome.
Predict patterns of inheritance for traits located on the sex chromosomes.
Describe how some common human genetic disorders are inherited.
Explain how changes in chromosomes can cause disorders in humans.
Check Your Understanding
How many alleles does an individual have for each gene/trait?
How do we predict the probability of traits being passed on to the next generation?
What do we call complexes of DNA wound around proteins that pass on genetic information to the next generation of cells?
Introduction
You might know someone who was born with a genetic disorder, such as cystic fibrosis or Down Syndrome. And you might have wondered how someone inherits these types of disorders. It all goes back to Mendel! Mendel’s rules laid the foundation for understanding the genetics of all organisms, including humans. We can apply Mendel’s rules to describe how many human traits and genetic disorders are inherited. Some disorders are caused by a recessive allele, while other disorders are caused by a single dominant allele. Therefore, we can draw a Punnett square to predict the number of offspring that may be affected with these diseases, just like we predicted for other traits in the previous lessons. Since Mendel’s time, we have also expanded our knowledge of inheritance and understand that genes are located on chromosomes. Now we can now explain special inheritance patterns that don’t fit Mendel’s rules.
Sex-linked Inheritance
What determines if a baby is a boy or a girl? Recall that you have 23 pairs of chromosomes, one pair of which are the sex chromosomes. Everyone has two sex chromosomes, X or Y, that determine our sex. Females have two X chromosomes, while males have one Y chromosome and one X chromosome. So if a baby inherits an X from the father and an X from the mother, it will be a girl. If the father’s sperm carries the Y chromosome, it will be a boy. Notice that a mother can only pass on an X chromosome, so the sex of the baby is determined by the father. The father has a 50 percent chance of passing on the Y or X chromosome, hence it is a 50 percent chance whether a child will be a boy or a girl.
One special pattern of inheritance that doesn’t fit Mendel’s rules is sex-linked inheritance, referring to the inheritance of traits which are due to genes located on the sex chromosomes. The X chromosome and Y chromosome carry many genes and some of them code for traits that have nothing to do with determining sex. Since males and females do not have the same sex chromosomes, there will be differences between the sexes in how these sex-linked traits are expressed.
One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot distinguish between red and green and often see these colors as shades of brown (Figure below). Boys are much more likely to be colorblind than girls. That’s because colorblindness is a sex-linked recessive trait. Boys only have one X chromosome, so if that chromosome carries the gene for colorblindness, they will be colorblind. As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal gene will mask the recessive colorblind gene. For a girl to be colorblind, she would have to inherit two genes for colorblindness, which is very unlikely. Many sex-linked traits are inherited in a recessive manner.
Figure 6.9
A person with red-green colorblindness would not be able to see the number.
A woman can be a carrier of colorblindness, however. A carrier appears normal but is capable of passing on a genetic disorder to her child. Carriers for colorblindness have a heterozygous genotype of one colorblind allele and one normal allele. We can use a Punnett square to predict the probability of a carrier passing on the trait to her children. For example, if a woman who is a carrier for colorblindness has children, her boys would have a 50% chance of being colorblind and her girls have a 50% chance of being carriers.
Xc X
X (carrier female)
XcX
(normal female)
XX
Y (colorblind male)
XcY
(normal male)
XY
Human Genetic Disorders
Some human genetic disorders are also X-linked or Y-linked, which means the faulty gene is carried on these sex chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; these chromosomes are known as autosomes or autosomal chromosomes.
Some genetic disorders are caused by recessive or dominant alleles of a single gene on an autosome. These disorders would then have the same inheritance pattern as any other dominant or recessive trait. An example of an autosomal recessive genetic disorder is cystic fibrosis. Children with cystic fibrosis have excessively thick mucus in their lungs which makes it difficult for them to breathe. The inheritance of this recessive allele is the same as any other recessive allele, so a Punnett square can be used to predict the probability that two carriers of the disease will have a child with cystic fibrosis.
F f
F (normal)
FF
(carrier)
Ff
f (carrier)
Ff
(affected)
ff
Another recessive trait that we mentioned previously was sickle cell anemia. A person with two recessive alleles for the sickle cell trait (aa) will have sickle cell disease. In this disease the hemoglobin protein is formed incorrectly and the person’s red blood cells are misshapen. A person who does not carry the sickle trait has a homozygous dominant genotype (AA). Remember the trait showed incomplete dominance, so a person who is heterozygous for the trait (Aa) would have some sickle-shaped cells and
some normal red blood cells.
You can also use a simple Punnett square to predict the inheritance of a dominant autosomal disorder, like Huntington’s disease. If one parent has Huntington’s disease, what is the chance of passing it on to their children? If you draw the Punnett square, you will see that there is a 50 percent chance of the disorder being passed on to the children. Huntington’s disease causes the brain’s cells to break down, leading to muscle spasms and personality changes. Unlike most other genetic disorders, the symptoms usually do not become apparent until middle age.
Genetic diseases can also be carried on the sex-chromosomes. An example of a recessive sex-linked genetic disorder is hemophilia. A hemophiliac’s blood does not clot, or clots very slowly, so he or she can easily bleed to death. As with colorblindness, males are much more likely to be hemophiliacs since the gene is on the X chromosome. Because Queen Victoria of England was a carrier of hemophilia, this disorder was once common in European royal families. Several of her grandsons were afflicted with hemophilia, but none of her granddaughters were affected by the disease, although they were often carriers. Because at the time medical care was very primitive, often hemophiliacs bled to death, and usually at a young age. Queen Victoria’s grandson Frederick died at age 3, and her grandson Waldemar died at age 11 (Figure below).
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