CHC Chapter 3 Study Guide

1. Describe the difference between dominant and recessive alleles. When do you see dominant and recessive traits in the offspring (during which generation)? Dominant alleles are always expressed whether there is one or two present in the DNA. Shows in first generation & second generation. Recessive alleles are only expressed if there are two of them. Only shows in second generation.

2. Name and describe the three exceptions to Mendel’s rules of heredity. Incomplete dominance – BLEND; each allele has some influence on a trait (eg: red flower crossed with white flower produces pink flowers) 3 phenotypes Codominance – 2 dominants (4 phenotypes) Sex-linked Traits – only on X One gene, many traits – one gene influences more than one trait Many genes, one trait – several genes work together to control one trait

3. How are sex cells different from other human body cells? Name the male and female sex cells. Sex cells carry ½ the number of chromosomes as body cells Egg - Female sex cell - XX Sperm - Male sex cell - XY

4. What are homologous chromosomes? Homologous chromosomes are a pair of chromosomes that carry the same sets of genes Extra Credit: How many chromosomes do humans have? 46

5. Why do sex linked disorders occur more often in males? Y chromosome does not carry all of the genes of the X chromosome Females have a backup gene if one is damaged, males do not

6. Describe the difference between genotype and phenotype. Which one determines the other? Genotype – inherited alleles for an organism - Actual genes inherited by an organism (2 letter code) Phenotype – organism’s appearance - physical appearance Genotype determines phenotype (& environment– too much sun changes skin color) Alleles – different versions of each trait (purple/white); represent with either capital or lowercase letter

7. What happens when a true-breeding plant self pollinates? When a true-breeding plant self-pollinates, all of its offspring will have the same trait as the parent Hybrid self-pollinates 3 dominant to 1 recessive (3:1 ratio)

8. How do the results of mitosis and meiosis differ? MitosisMeiosis # of divisions12 # of cells made24 # of stages4 (PMAT)8 (PMATPMAT) # of chromosomes (46)Same # as parent (46)Half # of parent (23) Type of cellsBodySex (egg/sperm) Why do this?Growth, Repair/Replace cells, Asexual reproduction Sexual reproduction Identical/NotIdenticalNon-identical

9. Define the following words. probability- mathematical chance that something will happen selective breeding- Organisms with desirable characteristics are mated pedigree- tool for tracing a trait through generations of a family

9. Define the following words. Homozygous or purebred- organism with two dominant or two recessive alleles (TT, tt) Heterozygous or hybrid- organism with one dominant & one recessive allele (Tt) On a Punnett Square – each box = 25%

10. Which type of dominance? A boy will inherit the disease if his mother has it A. Sex-linked traits – boy gets what mom has Organism can show black, tan, black & tan, or white phenotypes. B.Codominance – 4 phenotypes/2 dominants Organism can only have two possible phenotypes C.Complete – hybrid shows dominant trait Organism can have long, short, or medium tail. D. Incomplete – 3 phenotypes/blending

11. Fill in the chart for Complete Dominance GenotypePhenotypeDescription RRRoundHomozygous dominant RrRoundHeterozygous _________________________________ rrWrinkledHomozygous recessive

Practice Problems A cross is completed between two purebred organisms that have two different traits – one has attached earlobes and the other has unattached earlobes. When they mate, all their children have unattached earlobes. Which trait is dominant? Why? Unattached is dominant – only the dominant trait shows in the first generation.

Tall is dominant over short. Tt=tall What type of dominance is this? Complete (how I can tell: 2 phenotypes only or Tt shows the dominant trait) Complete a cross between a hybrid and short plant. What is the probability of them having short offspring? 50% (two boxes have tt=short, so 50%) T t tTttt

Red is dominant over white but heterozygous individuals show pink. Genotype of white flower ___________ Complete a cross between a red and pink flower. What is the probability of the offspring having genotype RR? _________ What is the probability of the offspring having pink flowers? ___________ What type of dominance is this? ________ rr R RRRRR rRrRr 50% Incomplete

Cows can be black, white, black & white spotted, or tan. What type of dominance is this? What is the recessive phenotype? Complete a cross between a purebred white cow and a spotted black & white cow. What is the probability of the offspring being spotted black & white? Codominance ii I W I B I W I B I W I B I W I W I W I W I W 50%