SCCS Life Science Name:
The Cell and Inheritance: Notes Period: Date:
Walter Sutton’s Work
You have seen that many organisms use sexual reproduction to reproduce by mixing and combining DNA to give offspring with new traits. As late as 1903, scientists knew that genes and traits were passed on from parents to offspring, but didn’t know exactly how this happened.
Working with grasshoppers, geneticist Walter Sutton observed that both grasshopper eggs and sperm had 12 chromosomes, which was exactly half the number of all other body cells in a grasshopper (they had 24). Once sperm and egg joined, the fertilized egg once again had 24 chromosomes. Sutton’s work led to a theory of how chromosomes and the traits they specify are passed from parents to offspring. It also explained how the sex cells were formed to give half the number of chromosomes.
Meiosis is the process of cell division that produces the sex cells egg and sperm (also called gametes) that have half the normal number of chromosomes as the rest of the cells in the body of the organism. In meiosis, specialized reproductive cells called germ cells first copy their chromosomes. Some chromosomes exchange sections before the germ cells divide so that the DNA is even more mixed. The exchange increases the diversity of the organisms. But at this point, the germ cells have the same number of chromosomes they started with. To get to half the number, they must divide a second time. At the end of meiosis, 4 new daughter cells have been created, each with different genetic information and half the number of chromosomes compared to the parent. These cells will turn into the gametes, sperm or egg.
A sperm and egg join to form a new organism in a process called fertilization. The fertilized egg is called a zygote and may develop further if it implants in the uterus of the female. After the zygote divides several times, the group of cells is called an embryo. The picture to the right describes the process of fertilization beginning with meiosis.
The X and Y sex chromosomes not only carry genes that determine the gender of the organism, but also a lot of other genes. Genes linked to a person’s gender are called sex-linked genes because they are found either on the X or the Y chromosome, but not both.
The concepts of dominant and recessive genes are the same with sex-linked genes as with all other genes. Two well-known diseases, hemophilia and red-green colorblindness, are caused by sex-linked genes. Colorblindness is a mild disability where a person sees certain colors such as red and green as shades of brown.
The gene for colorblindness is a RECESSIVE gene carried ONLY on the X chromosome. For example, a female might carry the recessive colorblind gene, but it is hidden by a dominant gene. This is the mother in the picture to the left. This woman is said to be a carrier - she carries the recessive gene for colorblindness but is not colorblind because she also has a dominant allele to hide it. However, she can pass that gene on to her children. Males are much more likely to be colorblind than females because the gene for colorblindness is carried only on the X chromosome and males have only one X chromosome! Compare the affected son in the picture with the carrier daughter. The affected son has the recessive X allele for colorblindness and no dominant allele to hide it. He has only a Y chromosome which does not carry the colorblind gene. Meanwhile, the girl does have the dominant X allele to hide her recessive colorblind allele.
Answer the questions below using your notes from class and the text below.
1. Working with grasshoppers, Walter Sutton noticed that the egg and sperm had _____________ the number of chromosomes compared to all other body cells in the insect.
2. Specialized sex cells called ____________________ are needed for sexual reproduction to occur. The male sex cell is called a ____________________ and the female sex cell is called an _______________.
3. Through the process of _________________________, the sex cells end up with half the number of chromosomes compared to normal body cells.
4. Circle the letter of the sentence that is true about meiosis. There could be more than one!
a. It gives the sex cells - egg and sperm.
b. 4 daughter cells are formed.
c. Parts of chromosomes exchange sections which increases genetic diversity.
d. Gametes have double the number of chromosomes compared to normal body cells.
5. Humans normally have 46 chromosomes in their body cells. How many chromosomes do the gametes have? _____________
6. The diagram to the below shows the 4 steps in meiosis in humans. Fill in the blanks with the correct number of chromosomes for each step.
a. like all other cells in humans, germ cells start with _________ chromosomes
b. chromosomes replicate, giving _________ chromosomes
c. first cell division happens, giving _________ chromosomes
d. second cell division, giving gametes with _________
chromosomes
7. Fill in the diagram below with the missing vocabulary:
a. Female sex cell: ________________________________
b. Male sex cell: ________________________________
c. Fertilization of an egg with a sperm results in the formation of a ___________________________
8. True or false: meiosis is when a sperm and egg join. ________________
9. _____________ _______________ genes are genes that linked to a person’s gender.
10. True or false: colorblindness is a recessive trait that occurs on the X chromosome. ________
11. List two reasons why colorblindness occurs much more in males than in females.
a. the gene for colorblindness is ___________________________________________________
b. males have _________________________________________________________________
12. A person has a sex-linked gene for a certain trait. They don’t exhibit that trait but can pass it on to their children. This person is said to be a _________________ for that gene.
Genetics Homework 2 - Answer Key
Worth 25 points:
1. half
2. gametes, sperm, egg
3. meiosis
4. a, b and c
5. 23
6. 46, 92, 46, 23
7. egg, sperm, zygote
8. false
9. sex linked
10. true
11. carried only by the X chromosome, only one X chromosome
12. carrier
1