1. Complex Inheritance
2019

2. 3/11 Objective: Finish Dragon Genetics poster and start guided reading on 12.3
DMA: Think of the variety of different heights you see in individuals in the classroom. Do you think the gene for height is controlled by just two alleles? Why or why not? If not, how is our height determined?

3. Motivational Monday

4. Other patterns of inheritance
Objectives:
Describe some of the exceptions to Mendel’s principles.
Explain the environment’s role in the way genes determine traits.

5. Codominance The phenotypes for both alleles are clearly expressed.
Examples: chicken feather color, human protein controlling blood cholesterol levels
Explain that in codominance, the phenotypes produced by both alleles are clearly expressed. In some varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.
Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers.
Ask: How is incomplete dominance different from codominance?
Answer: In incomplete dominance, the traits are blended. In codominance, both traits are distinctly expressed.

6. Many genes exist in more than two forms.
Multiple Alleles
Many genes exist in more than two forms.
Examples: human blood types, fur color in rabbits
Students are likely familiar with the concept of blood types. Ask them to identify the blood types they know. Explain that there being more than two alleles for a gene is common in a population. Make sure students understand, though, that any given individual in this population will have only two of those alleles.
Point out that in the case of human blood types, some alleles also show codominance. Tell students that the A and B alleles are codominant. A and B are each dominant over O. Explain that the Rh factor is inherited independently of the blood type alleles. Rh+ is dominant over Rh−.
To illustrate further how a gene can exist in more than two forms, write the symbols for four alleles for rabbit coat color on the board in order from the most dominant to the least dominant: C = full color, Cch = chinchilla color, Ch = Himalayan color, c = albino (no color). Have students make up genetic crosses for coat color in rabbits. If desired, have them exchange their proposed crosses with a partner who can then use Punnett squares to solve the problems.

7. Polygenic Traits
Many traits are produced by the interaction of several genes.
Examples: eye color in fruit flies, coat color in dogs
Traits typically show a wide variety of phenotypes.
Define polygenic traits for students: Polygenic traits are produced by the interaction of several genes.
Ask: How are multiple alleles different from polygenic traits?
Answer: Multiple alleles refers to different forms (alleles) for a single gene. Polygenic traits refers to multiple genes influencing a single trait.
Misconception Alert: Many students think that one gene is always responsible for one trait. Explain that such a case is actually rare. Most traits—such as hair and eye color in humans—are influenced by multiple genes.

8. Genes and the Environment
Environmental conditions can affect gene expression and influence genetically determined traits. .
Describe the example of the western white butterfly, which is found throughout western North America. Some people noticed over the years that western whites hatching in the summer had different color patterns on their wings than those hatching in the spring. Scientific studies showed the reason: Butterflies hatching in the shorter days of springtime had greater levels of pigment in their wings, making their markings appear darker than those hatching in the longer days of summer. In other words, the environment in which the butterflies develop influences the expression of their genes for wing coloration. Point out that pictures here are of buckeye butterflies, which show a similar pattern: They are darker in the summer than in the autumn.
Explain that the characteristics of any organism—whether plant, fruit fly, or human being—are not determined solely by the genes that organism inherits. Genes provide a plan for development, but how that plan unfolds also depends on the environment.
Ask for a volunteer to fill in the blanks verbally, reading the complete sentence.
Click to reveal the correct terms.
Ask students to consider in the case of the butterflies why being darker in the autumn than in the summer be useful. Explain that in order to fly effectively, the body temperature of the butterfly must be 28°C–40°C (about 84°F–104°F). Because the spring months are cooler in the West, greater pigmentation helps them reach the body temperature needed for flight. Similarly, in the hot summer months, less pigmentation enables the moths to avoid overheating.
Point out that “environment” refers to internal factors, too. For example, both men and women can have the genes for male pattern baldness, but baldness shows up more often in men because male hormones trigger the expression of the gene.

9. Some alleles are neither dominant nor recessive.
Incomplete Dominance
Some alleles are neither dominant nor recessive.
Incomplete dominance: One allele is not completely dominant over another.
Review with students the definitions of dominant and recessive.
Explain that four o’clock plants, whose flowers are shown here, have genes for flower color that don’t follow the strict dominance pattern that Mendel saw in different characteristics of pea plants. In four o’clock plants, the alleles for red and white flowers show incomplete dominance. Heterozygous (RR ) plants have pink flowers—a mix of red and white coloring.
Ask: Are the parent plants homozygous or heterozygous for flower color?
Answer: homozygous.
Ask: Are the offspring homozygous or heterozygous for flower color?
Answer: heterozygous
Ask: Could plants with pink flowers produce any offspring with red flowers?
Answer: Yes; each parent could give an “R” allele for flower color to an offspring, which would make that offspring homozygous for red flowers.

10. Karyotypes
Genome: the full set of genetic information that an organism carries in its DNA. (The analysis of any genome starts with chromosomes.)
Karyotype: shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.
Tell students: A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.
Explain that the image shows pairs of human chromosomes.
Ask: How many pairs of chromosomes does a typical human cell have?
Answer: 23 pairs
Tell students: This image, which shows all 23 pairs of human chromosomes arranged in decreasing size order, is a karyotype.
Ask: Why do our chromosomes come in pairs?
Answer: We begin life when a haploid sperm, carrying 23 single chromosomes, fertilizes a haploid egg, also with 23 single chromosomes. The resulting diploid cell develops into a new individual and carries the full complement of 46 chromosomes—two sets of 23.
Point out the difference in size of the X and Y chromosomes.
Click to highlight the X and Y chromosomes.
Make sure students note that the sex chromosomes are the only pair of chromosomes that do not match in size and gene content.

11. Females: XX – carries more than 1,400 genes
Sex Chromosomes
Two of the chromosomes in the human genome are sex chromosomes. 
 Females: XX – carries more than 1,400 genes
 Males: XY- carries about 158 genes
Tell students: The human Y chromosome is smaller and carries fewer genes than the human X chromosome.
Point out that more that 1200 genes are found on the X chromosome, some of which are shown in the figure.
Click to highlight the X chromosome.
Then, point out that the Y chromosome is much smaller and contains only about 140 genes, most of which are associated with male sex determination and sperm development.
Click to highlight the Y chromosome.

12. Autosomal Chromosomes
The other 22 pairs of non- sex chromosomes are called autosomal chromosomes or autosomes.
Tell students: The remaining 44 human chromosomes (22 pairs) that are not sex chromosomes are known as autosomal chromosomes, or autosomes.
Ask: What does the complete human genome consist of in terms of chromosomes?
Answer: The complete human genome consists of 46 chromosomes, including 44 autosomes and 2 sex chromosomes.
Tell students: To quickly summarize the total number of chromosomes present in a human cell—both autosomes and sex chromosomes—biologists write 46, XX for females and 46, XY for males.
Click to show the summary.

13. Transmission of Human Traits
Many human traits follow a pattern of simple dominance.
For example, the gene MC1R helps determine skin and hair color.
Some of MC1R’s recessive alleles produce red hair.
An individual with red hair usually has two of these recessive alleles.
Tell students: Human traits have many versions, and some are more dominant than others.
Provide students with the following example:
A gene known as MC1R helps determine skin and hair color.
Click to reveal the first bullet point.
Some of MC1R’s recessive alleles produce red hair.
Click to reveal the second bullet point.
An individual with red hair usually has two of these recessive alleles, inheriting a copy from each parent.
Click to reveal the third bullet point.
Dominant alleles for the MC1R gene help produce darker hair colors.

14. Codominant and Multiple Alleles
The alleles for many human genes display codominant inheritance.
Tell students: One example of codominant inheritance is the ABO blood group, which is determined by a gene with three alleles: IA, IB, and i.
Alleles IA and IB are codominant.
These genes code for proteins known as antigens on the surface of red blood cells.
Ask for a volunteer to identify an individual on the Blood Groups chart with both alleles IA and IB.
Click to verify the volunteer’s selection.
Tell students: Individuals with both alleles produce both A and B antigens.
Click to highlight the antigens.
Tell students: This makes the individuals’ blood type AB.
Click to highlight the blood type.
Tell students: The i allele is recessive.
Individuals with alleles IAIA or IAi produce only the A antigen, making them blood type A.
Ask: If an individual has IBIB or IBi alleles, what blood type is that individual?
Answer: blood type B
Click to validate the answer.
Tell students: Those individuals who are homozygous for the i allele (ii) produce no antigen and are said to have blood type O.
Ask: Why are there only four different phenotypes when there are six different genotypes?
Answer: The allele for no antigens, i, is recessive to IA and IB. Thus, two different genotypes—IAIA and IAi—result in the A phenotype, and two other genotypes—IBIB and IBi—result in the B phenotype.

15. Sex-Linked Inheritance
The X and Y chromosomes determine sex.
The genes located on them show sex linkage.
Colorblindness is a sex-linked trait
Tell students: A sex-linked gene is a gene located on a sex chromosome.
Genes on the Y chromosome are found only in males and are passed directly from father to son.
Genes located on the X chromosome are found in both sexes.
Help students explore sex-linked inheritance by examining a specific cross involving colorblindness. Tell students that doctors use images like the ones shown to test for colorblindness.
Have students construct a Punnett square to show a cross between a father with normal vision and a mother who is a carrier of the colorblindness trait.
Tell students to use the symbols XB to represent the dominant allele on the X chromosome and Xb for the recessive one.
Ask: What percent of the children have normal vision?
Answer: 75 percent
Ask: Are both daughters carriers? Explain.
Answer: No, only one daughter has the Xb allele.

16. X-Chromosome Inactivation
Because the X and Y chromosomes determine sex, the genes located on them show a pattern of inheritance called sex-linkage
Ex: Female calico cats are tri- colored.
The color of spots on their fur is controlled by a gene on the X chromosome.
Ask: If just one X chromosome is enough for cells in males, how does the cell “adjust” to the extra X chromosome in female cells?
Answer: In female cells, most of the genes in one of the X chromosomes are randomly switched off, forming a dense region in the nucleus known as a Barr body. Barr bodies are generally not found in males because their single X chromosome is still active.
As an example, explain that a gene that controls the color of a cat’s spots is located on the X chromosome.
Point out that female calico cats are tricolored and that the color of spots on their fur is controlled by a gene on the X chromosome.
Tell them that one X chromosome may have an allele for orange spots and the other X chromosome may have an allele for black spots.
In cells in some parts of the body, one X chromosome is switched off.
In other parts of the body, the other X chromosome is switched off.
As a result, the cat’s fur has a mixture of orange and black spots, like those in the cat shown.
Ask: How many different colors of spots can a male cat have?
Answer: Male cats, which have just one X chromosome, can have spots of only one color.
Draw the conclusion with students that, if the cat’s fur has three colors—white with orange and black spots, for example—you can almost be certain that the cat is female.

17. A pedigree chart shows the relationships within a family.
Human Pedigrees
A pedigree chart shows the relationships within a family. 
Ask: Given the complexities of genetics, how would you go about determining whether a trait is caused by a dominant or recessive allele and whether the gene for that trait is autosomal or sex-linked?
Answer: by applying Mendel’s basic principles of genetics
Ask: How would you analyze the pattern of inheritance followed by a particular trait?
Answer: You can use a chart that shows the relationships within a family. Such a chart is called a pedigree.
A pedigree shows the presence or absence of a trait according to the relationships among parents, siblings, and offspring.
It can be used for any species, not just humans.
Tell students: In a pedigree chart, a circle represents a female and a square represents a male. A horizontal line connecting a male and a female represents a marriage and a vertical line and a bracket connect the parents to their children. Last, tell them that a shaded circle or square indicates that a person expresses the trait, while one that is not shaded indicates that a person does not express the trait.

18. Human Pedigrees
The information from pedigree analysis makes it possible to determine the nature of genes and alleles associated with inherited human traits.
Explain to students that a pedigree helps trace inheritance patterns by looking at known phenotypes for a single trait. The information gained from pedigree analysis makes it possible to determine the nature of genes and alleles associated with inherited human traits.
Ask for volunteers to identify what each of the symbols means.
Ask: Where does the pedigree show that it is not possible for the gene to be recessive?
Answer: The gene cannot be recessive because if it were, the parents in the second generation who both show the phenotype would be homozygous recessive and could not have a child who shows the opposite phenotype.
Reinforce that pedigree analysis provides information on the nature of genes and alleles.
Ask: What are the genotypes of both parents on the left in the second row? How do you know?
Answer: Both are heterozygous, because they have a child with the recessive phenotype who must have received two copies of the recessive allele.
Explain to students that, based on a pedigree, you can often determine if an allele for a trait is dominant or recessive, autosomal or sex-linked.
Then, walk students through the pedigree.
Remind them that the allele for the white forelock trait is dominant, and on this chart that means the shape is colored blue.
Have students identify the phenotype and possible genotypes for each individual as you write them beside the appropriate symbols.
Activity: Have students draw out the pedigree, but this time with the trait of white forelock as recessive. Ask students to write out the genotypes and phenotypes of all of the individuals on the pedigree.
Go through the activity as a class.