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634 Chapter 19 NEL

• The chromosomal theory of inheritance:
  
  
  • Chromosomes carry genes, the units of heredity.
  
  
  • Each chromosome contains many different genes.
  
  
  • Paired chromosomes segregate during meiosis. Each sex cell or gamete has
    half the number of chromosomes found in a somatic cell.
  
  
  • Chromosomes assort independently during meiosis. This means that each
    gamete receives one member from each pair of chromosomes, and that each
    chromosome pair has no influence on the movement of any other
    chromosome pair.
  
  
  
  


• Females have two X chromosomes. Males have one X and one Y chromosome.


• Sex-linked traits are controlled by genes located on the sex chromosomes.
  A recessive trait located on the X chromosome is more likely to express itself in
  males than in females, since males need only one copy of the recessive allele
  while females need two.


• Female somatic cells can be identified by Barr bodies, which are actually
  dormant X chromosomes.

SUMMARY Chromosomes and Genetics

Section 19.1 Questions

1. Describe how the work of Walter S. Sutton and Theodor
   Boveri advanced our understanding of genetics.
   2.How do sex cells differ from somatic cells?
   3.Describe how Thomas Morgan’s work with Drosophila
   advanced the study of genetics.
   4.Identify two different sex-linked traits in humans.
   5.What are Barr bodies?
   6.A recessive sex-linked allele (h)located on the
   X chromosome increases blood-clotting time, causing
   hemophilia.
   (a) With the aid of a Punnett square, explain how a
   hemophilic offspring can be born to two normal
   parents.
   (b) Can any of the female offspring develop hemophilia?
   Explain.
   7. In humans, the recessive allele that causes a form of
   red–green colour-blindness (c)is found on the X
   chromosome.
   (a) Identify the F 1 generation from a colour-blind father
   and a mother who is homozygous for colour vision.
   (b) Identify the F 1 generation from a father who has colour
   vision and a mother who is heterozygous for colour
   vision.
   (c) Use a Punnett square to identify parents that could
   produce a daughter who is colour-blind.

Purpose Design Analysis

Problem Materials Evaluation

Hypothesis Procedure Synthesis

Prediction Evidence

To perform this investigation, turn to page 652.

Sex-Linked Traits

In this activity, you will cross Drosophila that carry genes for sex-
linked traits, using virtual fruit fly software. To determine if a trait
is sex-linked, you will perform two sets of crosses. In the first set
of crosses, you will confirm that a trait is sex-linked using males
and females with and without a trait. How will you set up the
crosses to get the data you will need? In the second set of
crosses, you will determine the phenotypic ratios in offspring of

the F 1 generation and observe the frequency of one trait in the

male and in the female offspring. What ratio would you expect

for a sex-linked trait?

INVESTIGATION 19.1 Introduction Report Checklist