Chapter 12

Chapter 12

by Nikki Chenevert

​Outline

Chromosome theory of inheritance: Mendelian genes have specific loci along chromosomes; chromosomes experience segregation and independent assortment.

Figure 12.2: separation of homologous pairs of chromosomes separates alleles. Arrangement at metaphase I.

Multiple Choice

Woman applied to the medico-genetic consulting center concerning the information about the risk of hemophilia of her son. Her husband has been suffering from this disease since birth. Woman and her parents are healthy (don't have hemophilia). Is the boy likely to have the disease in this family?

• All boys will be healthy

• All boys will be ill

• 50% of the boys will be ill

• 75% of the boys will be ill

Multiple Choice

According to the gene-chromosome theory, which statement is true?

• Genes are present only on human chromosomes.
• Genes are arranged in a linear sequence on a chromosome.
• Only one gene is needed to express a trait.
• Mutations occur only in sex cells.

Multiple Choice

In the human population, there are 100s of genes may code for the HLA protein.  How many of these alleles does an individual human possess?

• 200
• 4
• 2
• 100

Multiple Choice

Which of the following shows the correct scientific name for a fruit fly?

• Drosophila Melanogaster

• Drosophila melanogaster

• Drosophila melanogaster

• Drosophila Melanogaster

Multiple Choice

A researcher crossed a male Drosophila melanogaster having a grey body and long wings with a female D. melanogaster having a black body and apterous wings. The following distribution of traits was observed in the offspring.

What conclusion is supported by the data?

• The alleles for gray body and long wings are dominant.

• The alleles for gray body and long wings are recessive.

• Genes for the two traits are located on two different chromosomes, and independent assortment occurred.

• Genes for the two traits are located close together on the same chromosome, and crossing over occurred between the two gene loci.

​12.1 Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: Scientific Inquiry

1. Morgan’s Choice of Experimental Organism

   1. Fruit fly:

          i.      Only 4 pairs of chromosomes (3 Autosome, 1 Sex chromosome)

1.      Males XY; Females XX

          ii.      Observed a mutation of a male with white eyes (w),  compared to the wild type of red eyes (w⁺)

2. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair

   1. Typical 3 red: 1 white phenotype ratio,

Except all white eyed flies were males, so eye color must be linked to the sex (X) chromosome.  Figure 12.4

​12.2 Sex-linked genes exhibit unique patterns of inheritance

1. The Chromosomal Basis of Sex

   1. X Chromosome, carries 2,062 genes,

   2. Y Chromosome, only carries 78 genes, only in males

   3. Female (XX)

   4. Male (XY), ½ of sperm have an X, ½ of sperm have a Y

                                                              i.      Males determine sex of offspring, Figure 12.6

1. Other sex chromosomes in different species

2. Genes located on the sex chromosomes are sex-linked

3. X-linked genes are on the X chromosome

Multiple Choice

Suppose that in sheep, a dominant allele (B) produce black hair and a recessive allele (b) produce white hair. If you saw a black sheep, you would be able to identify

• its phenotype for hair color

• its genotype for hair color

• the genotype of only one of its parents

• the phenotype of both of its parents

Multiple Choice

A male fruit fly (Drosophila melanogaster) with red eyes and long wings was mated with a female with purple eyes and vestigial wings. All of the offspring in the F1 generation had red eyes and long wings. These F1 flies were test crossed with purple-eyed, vestigial-winged flies. Their offspring, the F2 generation, appeared as indicated below.

If in the F1 and F2 generations the same characteristics appeared in both males and females, it would be safe to assume that these traits for eye color and wing length are...

• are sex-linked

• are sex-influenced characteristics

• are autosomal characteristics

• follow the Mendelian rule of independent assortment

Multiple Choice

The pedigree of a family with a history of a particular genetic disease is shown below. Squares represent males and circles represent females. Shaded symbols represent those who have the disease.

If Individual 2 were to marry a woman with no family history of the disease, which of the following would most likely be true of their children?

• All of the children would have the disease.

• None of the children would have the disease

• Only the sons would have the disease

• All of the sons would be carriers of the disease

Multiple Choice

The pedigree of a family with a history of a particular genetic disease is shown below. Squares represent males and circles represent females. Shaded symbols represent those who have the disease

If Individual 6 marries a woman with the disease, what is the probability that their first child will have the disease?

• 0

• 25%

• 50%

• 75%

Multiple Choice

In humans, hemophilia is an X-linked recessive trait. If a man and a woman have a son who is affected with hemophilia, which of the following is definitely true?

• The mother carries an allele for hemophilia
• The father carries an allele for hemophilia
• The father is afflicted with hemophilia
• both parents carry an allele for hemophilia

​Inheritance of X-linked Genes (Figure 12.7)

1. Fathers pass to daughters, not to sons

2. Mothers pass to daughters and sons

3. Females have two copies, but males only have one copy

                                                              i.      Female heterozygotes can be carriers of X-linked recessive traits

                                                            ii.      More males have x-linked recessive disorders, because males only have 1 X, so trait is expressed. Females have another X that can mask the expression of a recessive trait

                                                          iii.      If mutated genes are on X chromosomes, females have another X chromosome that can carry the dominant allele and mask the expression of the mutant recessive gene.

                                                          iv.      Ex. Red-green Color blindness, hemophilia, Duchenne muscular dystrophy

2. X Inactivation in Female Mammals

   1. To equalize the effect of x-linked gene products in male and females, one X chromosome is inactivated

                                                              i.      Barr body: the inactive X chromosome (with the exception of about 3 dozen genes)

                                                            ii.      This happens very early on in development (about 200 cells big) and is random within all these early cells.  From then on all cells that are made from that cell will have the same chromosome condensed and inactive.

                                                          iii.      So in an adult female, some regions of the body are expressing different x chromosomes.  Calico Cat

​

2. X Inactivation in Female Mammals

   1. To equalize the effect of x-linked gene products in male and females, one X chromosome is inactivated

                                                              i.      Barr body: the inactive X chromosome (with the exception of about 3 dozen genes)

                                                            ii.      This happens very early on in development (about 200 cells big) and is random within all these early cells.  From then on all cells that are made from that cell will have the same chromosome condensed and inactive.

                                                          iii.      So in an adult female, some regions of the body are expressing different x chromosomes.  Calico Cat

Multiple Choice

Examples of non-Mendelian forms of inheritance:

• Incomplete dominance

• Codominance

• Polygenic inheritance

• Multiple alleles

• All of the above

Multiple Choice

What are some sex-linked traits?

• color blindness

• hemophilia

• Both answers are right

Multiple Choice

What is a Barr body?

• A Barr body is an X chromosome that is condensed and visible.

• A Barr body is an X chromosome that is not condensed nor visible.

​12.3 Linked genes tend to be inherited together because they are located near each other on the same chromosome

1. Chromosomes contain multiple genes

2. Genes located near each other on the same chromosome (

3. Genes on different chromosomes assort independently

1. How Linkage Affects Inheritance

   1. Linked genes are inherited together and break Mendel’s Law of Independent Assortment (Figure 12.9)

   2. Some of the nonparental phenotypes were produced by genetic recombination.

   3. Genetic recombination: the combination of alleles for different genes in two parental individuals become shuffled into new combinations in offspring (crossing over)

​12.3 Linked genes tend to be inherited together because they are located near each other on the same chromosome

1. Chromosomes contain multiple genes

2. Genes located near each other on the same chromosome (

3. Genes on different chromosomes assort independently

1. How Linkage Affects Inheritance

   1. Linked genes are inherited together and break Mendel’s Law of Independent Assortment (Figure 12.9)

   2. Some of the nonparental phenotypes were produced by genetic recombination.

   3. Genetic recombination: the combination of alleles for different genes in two parental individuals become shuffled into new combinations in offspring (crossing over)

Multiple Choice

Genetic recombination is the production of offspring - - - 

• with new combinations of genes
• with similar genes as their parent
• that are completely different from the parents
• that are super cute and irresistible

Multiple Choice

If results of recombination do NOT follow Mendel's Law of Independent Assortment, genes are likely to be - - - 

• shuffled during meiosis
• linked
• inherited
• on separate chromosomes

Multiple Choice

Linked genes tend to be inherited together, like freckles and red hair, because they - - - 

• compliment each other well
• are on chromosomes close together
• are on the same chromosome 
• tend to be adaptations best suited for the environments of the organism

​Recombination and Linkage

1. Recombination of Unlinked Genes: Independent Assortment of Chromosomes

                                                              i.      50% offspring match parental phenotypes, 50% are recombinant offspring, genes on different chromosomes

1. Recombination

                                                              i.      If the majority of offspring have parental phenotypes, the genes are linked.

                                                            ii.      Recombination is a result of crossing over during prophase I of meiosis I

1. New Combinations of Alleles: Variation for Natural Selection

​ 

1. Mapping the Distance Between Genes Using Recombination Data

                                                              i.      Genetic Map is a list of loci along a chromosome

                                                            ii.      Based on frequency of recombination is a linkage map

                                                          iii.      The closer two genes are, the more likely they will be inherited together

                                                          iv.      If they are far enough apart, multiple crossover events can lead to independent assortment.

                                                            v.      Crossing over disrupts linkages

​12.4 Alterations to chromosome number and or structure cause some genetic disorders

1. Abnormal Chromosome Number

   1. Nondisjunction: failure for homologous chromosomes or sister chromatids to separate during meiosis

   2. Aneuploidy: abnormal chromosome numbers due to nondisjunction during gamete formation

                                                              i.      Trisomic: 2n + 1 chromosome, 3 of 1 chromosome

                                                            ii.      Monsomic: 2n – 1 chromosome, only 1 of 1 chromosome

                                                          iii.      Polyploidy: more than two complete sets of chromosomes

1.      Common in plants

​Alterations to Chromosome Structure

1. Deletions: chromosomal fragment is lost

2. Duplications: a chromosomal fragment is repeated

3. Inversion: chromosomal fragment is removed reversed and inserted

4. Translocation: a chromosomal fragment is removed from one chromosome and added to a nonhomologus chromosome

​Alterations to Chromosome Structure

1. Deletions: chromosomal fragment is lost

2. Duplications: a chromosomal fragment is repeated

3. Inversion: chromosomal fragment is removed reversed and inserted

4. Translocation: a chromosomal fragment is removed from one chromosome and added to a nonhomologus chromosome

​Human Disorders Due to Chromosomal Alterations

1. Down Syndrome (Trisomy 21)

2. Aneuploidy of Sex Chromosomes

                                                              i.      XXY: Kleinfelter Syndrome: male sex organs, but sterile, female body characteristics, subnormal inteligence

                                                            ii.      XYY: normal sexual development, no well-defined syndrome

                                                          iii.      XXX: Trisomy X: taller than average, learning disabilities, fertile

                                                          iv.      X: Turner Syndrome: Sterile, no mature sex organs, normal inteligence