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BIO1.LS3: Heredity: Inheritance and Variation of Traits

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BIO1.LS3: Heredity: Inheritance and Variation of Traits

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1) Model chromosome progression through meiosis and fertilization in order to argue how the processes of sexual reproduction lead to both genetic similarities and variation in diploid organisms.

Compare and contrast the processes of sexual and asexual reproduction, identifying the advantages

and disadvantages of each.

2) Explain how protein formation results in phenotypic variation and discuss how changes in DNA can lead to somatic or germ line mutations.

3) Through pedigree analysis, identify patterns of trait inheritance to predict family member genotypes. Use mathematical thinking to predict the likelihood of various types of trait transmission.

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A regular human cell has 46 chromosomes: 44 autosomes, which come in pairs, and 2 sex chromosomes, which specify whether someone is male (usually XY) or female (usually XX).

The pairs of autosomes are called "homologous chromosomes." Homologous chromosomes have all of the same genes arranged in the same order, but there are small differences in the DNA letters of the genes.

When cells divide to make more cells (mitosis) or reproductive cells (meiosis), and when reproductive cells join to make a new individual (fertilization), it is important that the new cells get the proper number of chromosomes. Read on to learn more about these processes.

Before a cell divides to make two cells, it copies all of its chromosomes. These copies, called sister chromatids, are identical. Until the cell divides, the identical copies stay connected with each other by their middles (centromeres.) When the cell divides, the copies are pulled apart, and each new cell gets one identical copy of each chromosome.

This type of cell division is called mitosis, and it produces cells with a total of 46 chromosomes. Beginning soon after fertilization (see below), all of the cells in your body were made this way. Thus, every cell in your body has an identical set of chromosomes.

When egg and sperm form, they go through a special type of cell division called meiosis. One purpose of meiosis is to reduce the number of chromosomes by half. The other is to create genetic diversity.

Meiosis begins like mitosis: the cell copies each chromosome. But unlike in mitosis, homologous chromosome pairs line up and exchange pieces-a process called recombination. Remember, homologous chromosomes have the same genes but with slight differences. Recombination increases genetic diversity by putting pieces of slightly different chromosomes together.

Next, the newly recombined homologous chromosomes are divided into two daughter cells. Then the sister chromatids are pulled apart into a total of four reproductive cells. Each of these cells has one copy each of 23 chromosomes, all with a unique combination of gene variations.

Egg and sperm cells have just 23 chromosomes each. That's half as many chromosomes as regular cells. Through the process of fertilization, egg and sperm join to make a cell with 46 chromosomes (23 pairs), called a zygote.

For each chromosome pair, one homologous chromosome came from each parent. They have the same genes arranged in the same order, but there are small variations in the DNA letters of those genes.

From here, the process begins again. Mitosis builds a person with an identical set of chromosomes in every cell. And meiosis generates reproductive cells with new combinations of gene variations.

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A regular human cell has 46 chromosomes: 44 autosomes, which come in pairs, and 2 sex chromosomes, which specify whether someone is male (usually XY) or female (usually XX). The pairs of autosomes are called "homologous chromosomes." Homologous chromosomes have all of the same genes arranged in the same order, but there are small differences in the DNA letters of the genes.

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Read the site and understand the vocabulary used throughout the lesson.

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Asexual and Sexual reproduction on a cellular level

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Asexual and Sexual reproduction on a cellular level

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All 50 chromosomes are in the asexual offspring, while 23 from mom and 23 from dad are in the sexual offspring.

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Compare and contrast types of reproduction

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Compare and contrast types of reproduction

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Watch the video on phenotypes and proteins in genes.

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Watch the video for a general understanding of Meiosis.

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Meiosis: Where the Sex Starts - Crash Course Biology #13

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Read the article to get a more detailed description of the mutations.

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Germline mutations are DNA changes inherited during conception. Somatic mutations happen after conception to cells other than the egg and sperm.

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Look at the graphic and go through the differences of somatic and germ-line.

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This video is over Pedigrees in general. Watch it for a basic understanding of pedigrees.

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Take a look at the pedigree and use the key to see how traits are passed.

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Read this article to understand how Punnett Squares use math to track heritability of traits.

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Punnett Squares

A Punnett square is a chart that allows you to easily determine the expected percentage of different genotypes in the offspring of two parents. An example of a Punnett square for pea plants is shown in Figure below. In this example, both parents are heterozygous for flowercolor ( Bb). The gametes produced by the male parent are at the top of the chart, and the gametes produced by the female parent are along the side. The different possible combinations of alleles in their offspring are determined by filling in the cells of the Punnett square with the correct letters (alleles). At the link below, you can watch an animation in which Reginald Punnett, inventor of the Punnett square, explains the purpose of his invention and how to use it. http://www.dnalc.org/view/16192-Animation-5-Genetic-inheritance-follows-rules-.html

An explanation of Punnett squares can be viewed at http://www.youtube.com/watch?v=D5ymMYcLtv0 (25:16). Another example of the use of a Punnett square can be viewed at http://www.youtube.com/watch?v=nsHZbgOmVwg (5:40).

This Punnett square shows a cross between two heterozygotes, Bb. Do you know where each letter (allele) in all four cells comes from? Two pea plants, both heterozygous for flower color, are crossed. The offspring will show the dominant purple coloration in a 3:1 ratio. Or, about 75% of the offspring will be purple.

Predicting Offspring Genotypes

In the cross shown in Figure above, you can see that one out of four offspring (25 percent) has the genotype BB, one out of four (25 percent) has the genotype bb, and two out of four (50 percent) have the genotype Bb. These percentages of genotypes are what you would expect in any cross between two heterozygous parents. Of course, when just four offspring are produced, the actual percentages of genotypes may vary by chance from the expected percentages. However, if you considered hundreds of such crosses and thousands of offspring, you would get very close to the expected results, just like tossing a coin.

Predicting Offspring Phenotypes

You can predict the percentages of phenotypes in the offspring of this cross from their genotypes. B is dominant to b, so offspring with either the BB or Bb genotype will have the purple-flower phenotype. Only offspring with the bb genotype will have the white-flower phenotype. Therefore, in this cross, you would expect three out of four (75 percent) of the offspring to have purple flowers and one out of four (25 percent) to have white flowers. These are the same percentages that Mendel got in his first experiment.

Determining Missing Genotypes

A Punnett square can also be used to determine a missing genotype based on the other genotypes involved in a cross. Suppose you have a parent plant with purple flowers and a parent plant with white flowers. Because the b allele is recessive, you know that the white-flowered parent must have the genotype bb. The purple-flowered parent, on the other hand, could have either the BB or the Bb genotype. The Punnett square in Figure below shows this cross. The question marks (?) in the chart could be either B or b alleles.

Punnett Square: Cross Between White-Flowered and Purple-Flowered Pea Plants. This Punnett square shows a cross between a white-flowered pea plant and a purple-flowered pea plant. Can you fill in the missing alleles? What do you need to know about the offspring to complete their genotypes?

Can you tell what the genotype of the purple-flowered parent is from the information in the Punnett square? No; you also need to know the genotypes of the offspring in row 2. What if you found out that two of the four offspring have white flowers? Now you know that the offspring in the second row must have the bb genotype. One of their b alleles obviously comes from the white-flowered ( bb) parent, because that's the only allele this parent has. The other b allele must come from the purple-flowered parent. Therefore, the parent with purple flowers must have the genotype Bb.

Punnett Square for Two Characteristics

When you consider more than one characteristic at a time, using a Punnett square is more complicated. This is because many more combinations of alleles are possible. For example, with two genes each having two alleles, an individual has four alleles, and these four alleles can occur in 16 different combinations. This is illustrated for pea plants in Figure below. In this cross, known as a dihybrid cross, both parents are heterozygous for pod color ( Gg) and pod form ( Ff).

Punnett Square for Two Characteristics. This Punnett square represents a cross between two pea plants that are heterozygous for two characteristics. G represents the dominant allele for green pod color, and g represents the recessive allele for yellow pod color. F represents the dominant allele for full pod form, and f represents the recessive allele for constricted pod form.

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Explore More I

Use this resource to answer the questions that follow.

1. What is a Punnett square?

2. What do the boxes in a Punnett square represent?

3. What is the size of a Punnett square used in a dihybrid cross?

4. Define the following terms: alleles, genotype, phenotype, genome.

Review

1. What is a Punnett square? How is it used?

2. Draw a Punnett square of an Ss x ss cross. The S allele codes for long stems in pea plants and the s allele codes for short stems. If S is dominant to s, what percentage of the offspring would you expect to have each phenotype?

3. What letter should replace the question marks (?) in this Punnett square? Explain how you know.

4. How do the Punnett squares for a monohybrid cross and a dihybrid cross differ?

5. What are the genotypes of gametes of a AaBb self-pollination?

6. Mendel carried out a dihybrid cross to examine the inheritance of the characteristics for seed color and seed shape. The dominant allele for yellow seed color is Y, and the recessive allele for green color is y. The dominant allele for round seeds is R, and the recessive allele for a wrinkled shape is r. The two plants that were crossed were F1 dihybrids RrYy. Identify the ratios of traits that Mendel observed in the F2 generation. Create a Punnett square to help you answer the question.

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A Punnett square is a chart that allows you to easily determine the expected percentage of different genotypes in the offspring of two parents. An example of a Punnett square for pea plants is shown in Figure below. In this example, both parents are heterozygous for flowercolor ( Bb).

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Heredity: inheritance and variation of traits

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Heredity: inheritance and variation of traits

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Take a walk through this lesson on heredity, traits, and reproduction. Follow the arrows!!