Bsci110b Chapter 15 Concept Check

 Which one of Mendel’s laws relates to the inheritance of alleles for a single character?  Which law relates to the inheritance of alleles for two characters in a dihybrid cross? Mendel derived the  from following a single character, which states that the two alleles for a heritable character segregate during gamete formation and end up in different gametes.  Using a dihybrid cross Mendel developed the , which states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.   

What is the physical basis of Mendel’s laws? The physical basis for the law of segregation is the separation of homologs in .  The physical basis for the law of independent assortment is the alternative arrangements of homologous chromosome pairs in  .

Propose a possible reason that the first naturally occurring mutant fruit fly Morgan saw involved a gene on a sex chromosome. To show the mutant phenotype, a male needs to possess  mutant allele.  If this gene had been on a pair of , two mutant alleles would have had to be present for an individual to show the mutant phenotype, a much less probable situation.  

 A white-eyed female Drosophila is mated with a red-eyed (wild-type) male, the reciprocal cross of that is shown.  What phenotypes and genotypes do you predict for the offspring? Because the gene for this eye-color character is located on the X chromosome, all female offspring will be red-eyed ; all male offspring will inherit a Y chromosome from the father and be white-eyed .

Neither Tom nor Rhoda has Duchenne muscular dystrophy, but their firstborn son does have it.  What is the probability that a second child of this couple will have the disease?  What is the probability if the second child is a boy?  A girl? There is a ¼ chance that the firstborn son does have it, a   chance that the child will inherit a Y chromosome from the father and be male multiplied by a   chance that he will inherit the X carrying the disease allele from his mother.  If the child is a boy, there is a chance he will have the disease; a female would have zero chance (but  chance of being a carrier).

  During early embryonic development of female carriers for color blindness, the normal allele is inactivated by chance in about half the cells.  Why, then, aren’t 50% of female carriers color-blind? The cells in the eye responsible for color vision must come from multiple cells in the early embryo.  The descendants of  of those cells express the allele for normal color vision and  the allele for color blindness.  Having  the number of mature eye cells expressing the normal allele must be sufficient for normal color vision.

When two genes are located on the same chromosome, what is the physical basis for the production of recombinant offspring in a testcross between a dihybrid parent and a double-mutant (recessive) parent? Crossing over during  in the  parent produces some gametes with recombinant genotypes for the two genes.  Offspring with a recombinant phenotype arise from fertilization of the recombinant gametes by homozygous recessive gametes from the double-mutant parent.

Genes A, B, and C are located on the same chromosome.  Testcrosses show that the recombination frequency between A and B is 28% and between A and C is 12%.  Can you determine the linear order of these genes? Explain. . To determine which possibility is correct, you need to know the recombination frequency between  and .

More common than completely polyploidy animals are mosaic polyploids, animals that are diploid except for patches of polyploidy cells.  How might a mosaic tetraploid, an animal with some cells containing four sets of chromosomes arise? At some point during development, one of the embryo’s cells may have  to carry out mitosis after duplicating its chromosomes.  Subsequent normal cell cycles would produce genetic copies of this tetraploid cell.

About 5% of individuals with Down syndrome have a chromosomal translocation in which a third cop of chromosome 21 is attached to chromosome 14.  If this translocation occurred in a parent’s gonad, how could it lead to Down syndrome in a child? In meiosis, a combined 14-21 chromosome will behave as one chromosome.  If a gamete receives the combined 14-21 chromosome and a normal copy of chromosome 21, trisomy 21 will result when this gamete combines with a normal gamete during .

The ABO blood type locus has been mapped on chromosome 9.  A father who has type AB blood and a mother who has type O blood have a child with trisomy 9 and type A blood.  Using this information, can you tell in which parent the nondisjunction occurred? .  The child can be either I^AI^A or I^Aii.  A sperm of genotype I^AI^A could result from nondisjunction in the father during , while an egg with the genotype ii could result from nondisjunction in the mother during either  or .

Gene dosage, the number of active copies of a gene, is important to proper development.  Identify and describe two processes that establish the proper dosage of certain genes. Inactivation of an  chromosome in females and .  Because of X inactivation, the effective dose of genes on the X chromosome is  in males and females.  As a result of genomic imprinting,  allele of certain genes is phenotypically expressed.  

 Reciprocal crosses between two primrose varieties, A and B, produced the following results: A female X B male => offspring with all green (nonvariegated) leaves.  B female X A male => offspring with spotted (variegated) leaves.  Explain. The genes for leaf coloration are located in  within the cytoplasm.  Normally, only the  parent transmits plastids genes to offspring.  Since variegated offspring are produced only when the female parent is of the  variety, we can conclude that variety B contains both the wild-type and mutant alleles of pigment genes, producing variegated leaves.

 Mitochondrial genes are critical to the energy metabolism of cells, but mitochondrial disorders caused by mutations in these genes are generally not lethal.  Why not? The situation is similar to that for chloroplasts.  Each cell contains numerous mitochondria, and in affected individuals, most cells contain a variable mixture of normal and mutant mitochondria.  The  mitochondria carry out enough cellular respiration for survival.