Chapter 26 ……Inheritance Of Traits

 

Students should be able to:

 

·        Compare the number of chromosomes in sex cells and body cells

·        Distinguish between dominant and recessive genes

·        Describe how different gene combinations result from fertilization and how traits are passed to offspring .

·        Discuss the purpose of a punnett square

·        Compare expected results and observed results.

·        Explain the importance of Gregor Mendel’s work

·        Solve genetic problems

 

Gregor Mendel's Contribution to the Subject of Inheritance
Mendel observed how specific traits of the garden pea were transmitted from generation to generation. Mendel kept precise records of the thousands of offspring (and their characteristics) produced in his crosses. He then established mathematical probabilities and explanations to validate his observations.

Although others had studied inheritance, Mendel's educational experiences in math and observing plant variation helped him design and analyze his experiments carefully. Mendel:

• Chose a good organism that had a number of "true breeding" traits easy to observe.
• Designed the experiments carefully. Mendel took plants from true breeding parents (P generation) , to first generation (F₁ hybrids), and then self-crossed the first generation offspring to form a second generation (F₂).
• Obtained large sample sizes for good data analysis

Mendel's research led to the following conclusions, two of which are presented as Mendel's Principles:

Mendel's Statements about Inheritance

1. There are alternative forms (or variations) of genes, the "units" that determine inherited traits. The alternative forms of a gene are now called alleles. To relate this to what we know about homologous chromosomes, the alleles are located at the same locus on homologous chromosomes. (Specifically, we inherit the alleles for a gene, not the gene).
2. An individual will have 2 alleles for each inherited trait. The 2 alleles may be the same, or they may be different. If the two alleles are the same, the individual will be homozygous for that trait. If the two alleles are different, the individual will be heterozygous for the trait.
   
   When the two alleles for a gene pair are different from each other, one will be expressed, and the second will not affect the organism's appearance. The allele always expressed is said to be dominant, and the one that may not be expressed is recessive.
   
   Note: These statements are true for the traits tested in Mendel's peas and for many genes, but are not universally true. Many genes have alleles that are equally expressed, as we shall see, and there are genes that have more than 2 alleles within the population.
3. Gametes have just one allele for each trait, because the allele (gene) pairs are separated (or segregated) during meiosis I when homologous chromosomes pair and then separate. 50% of the gametes receive one allele and 50% of the gametes receive the alternative allele when the alleles are heterozygous. (And as Mendel proposed, fertilization results in restoring the pairs of alleles for the next generation).
   
   This statement ultimately resulted in Mendel's Principle of Segregation: Pairs of genes segregate during the formation of gametes (Meiosis), so that each gamete has one of each gene pair (one allele) but not both. Fertilization restores the gene pairs (on the homologous chromosomes).
   
   Mendel demonstrated his Principle of Segregation with many monohybrid crosses, looking at one characteristic at a time.

Some terms used in Mendelian Inheritance Tests

True Breeding

• A plant that produces offspring with the same characteristics. The parental generation is a true-breeding generation.

Cross Breeding

• A cross between different parental types
• Offspring produced by cross breeding are called Hybrids

F₁ Generation

• The first generation
• Generally first generation offspring are bred among themselves to produce the second generation. In Mendel's pea plants they self-fertilized.

F₂ Generation

• The second generation
• Mendelian ratios are based on second generation results

Punnett Square

• A method of visualizing Inheritance crosses

Gene

• The physical unit of heredity; the instructions for producing a specific characteristic or trait. For example, the characteristic or gene may be flower color. The alternative forms a gene can have would be the specific flower colors.
• Since diploid organisms have two sets of chromosomes (the homologous chromosome pairs), most "genes" are paired, often called the gene pair

Alleles

• The alternative forms or variations a gene can have, such as brown or blue for eye color, or red or white for flower color. The word trait is often used to describe the specific alleles, but trait is also used to describe the gene, too.
• A diploid individual will have two alleles for each gene locus.
• Within a population there can be more than two alleles for a gene, but only two alleles will be present in any one diploid individual.

Locus

• The region on a chromosome where a gene is located.
• The alleles of a gene are located at equivalent places (loci) on the homologous chromosomes

Homozygous

• The 2 alleles for a gene are the same in an individual

Heterozygous

• The 2 alleles for a gene are different in an individual

Dominant Allele (loosely and incorrectly called a dominant gene)

• An allele which is always expressed, whether it is homozygous or heterozygous.
• A dominant allele masks or covers the expression of its alternative allele.

Recessive Allele

• An allele which is masked by the presence of its alternative.
• A recessive allele will be expressed only when it is homozygous, (when the dominant allele is absent)

Phenotype

• The observable traits of an individual

Genotype

• The specific genetic makeup of an individual, or total combination of alleles present, both those expressed and those not expressed.