Patterns of Inheritance Chapter 13

Patterns of Inheritance Look around you. Variation can be seen everywhere you look. Your classmates have different color eyes, hair, and skin. They are different heights and have different bone structures. We are going to explore the differences and how the environment and genetics cause these.

Heredity and the Environment While you may have inherited your height and body build from your parents, the environment also affects these traits. Nutrition and exercise are two environmental factors that can affect height and body build.

Mendel Mendel is considered to be the father of genetics. He conducted carefully controlled experiments with pea plants and discovered the basic principles of heredity. Mendel was a friar at the Augustinian monastery in Brno, Czechoslovakia.

Mendel was experimenting with flowers in the monastery's gardens, trying to develop new color variations. It was these experiments that led to his experiments in hybridization. But Mendel was not alone in his field... ever since humans began domesticating plants and animals (at least 10,000 years ago), we wondered how traits were passed from parent to offspring. One belief was that traits were stored as 'particles' in the parts of each parent's body and 'blended' in the offspring. This theory left many questions untackled, however.

In the 18th and 19th centuries, scientists tried to unravel these questions, but their means returned inconclusive results. Unlike the others, Mendel studied only one trait at a time. Because of this, he was the first to be able to describe the relations between parents and children with mathematical symbols.

Useful Vocabulary Genetics – the study of inheritance. Character- heritable trait such as flower color or hair color. Trait – the variant for the character – purple flowers, red hair.

Mendel’s Experiments Mendel chose a common garden pea (Pisum) for his first experiments in hybridization. These plants exhibited what are now called "Mendelian Traits" - traits which occur in a very simple form. (These are traits that only have two forms- green peas or yellow peas; red flowers or white flowers).

When Mendel started his experiments, he immediately noticed that when breeding two peas, a particular variation of a trait in one pea (for example, the greenness of a pea) would not appear in the next generation. However, in the following generation, when breeding the offspring together, this variation would appear again. He concluded that the traits were being "masked" in the second generation, to be exhibited again in the third.

When two plants breed, the variations of their traits are combined When two plants breed, the variations of their traits are combined. The combination can only be explained by assuming that, for each trait, there is space for two pieces of "information" describing the variation. Say we have a pea which is "purebred" to green (that is, when it is bred with itself, it will create only green peas), and another which is "purebred" to yellow. If we breed a purebred green pea and a purebred yellow pea, and our result is all yellow peas, we can say that the "green" variation has been lost. However, since in the next generation, yellow peas appear again, we must instead say that the "green" variation was masked, not lost.

Vocabulary Traits that “masked” other traits, are described as dominant. Traits that are “masked” are said to be recessive. The standard way of labeling the variation information of a trait in a particular organism is using two letters. Capital letters represent information which is dominant. Lowercase letters represent the recessive. The letter being used describes a variation (usually the recessive) of the trait.

Generations P generation is the parent generation. F1 is the first filial generation or the offspring of the P generation. F2 is the second filial generation or the offspring of the F1 generation.

Generations

Mendel’s Law of Segregation The Law of Segregation states that the members of each pair of alleles separate when gametes are formed. A gamete will receive one allele or the other. In other words, two alleles for a character are packaged into separate gametes (one from the maternal line and one from the paternal line.)

Mendel’s Law of Independent Assortment The Law of Independent assortment states that two or more pairs of alleles segregate independently of one another during gamete formation. Or- alleles for a characteristic divide up among gametes (during meiosis) independently of one another.

Alleles Alleles can be dominant or recessive. Individuals can be homozygous dominant (BB), homozygous recessive (bb) or heterozygous (Bb). In homozygous dominant and heterozygous individuals the dominant trait is expressed. For a recessive trait to be expressed, the individual must be homozygous recessive.

Phenotype and Genotype The phenotype is the exhibited or expressed traits. The genotype is the genetic make-up. The genotype and phenotype are the same for homozygous individuals. Heterozygotes are more difficult to determine because they “carry” the recessive trait even though it is not expressed.

Probability and Punnett Squares We can determine the probability of traits shown in potential offspring of two individuals by using a Punnett Square. The probabilities are just like tossing a coin.

Probability Probability ranges from 1 – very certain an event will happen to 0 – the event will not happen. What is the probability that a coin when tossed will land on heads? What is the probability that when tossed two coins will both land on heads. ½ x ½ = ¼

Using Probability to Solve Genetic Problems We can use the Punnet square to calculate the number of offspring likely to result from the individuals of known genotype. Draw a 4-square Punnet square and determine the likely numbers of offspring for the following crosses. TT x tt and Bb x bb

T = tall, t = short B = brown eyes, b = blue

Dihybrid Cross T = tall, t = short B = brown eyes, b = blue TtBB x ttBb FOIL is used to determine the genotypes. Using the above parents, calculate the probable numbers of offspring with the traits that result from this cross.

TtBB x ttBb  

Tri-Hybrid Cross If R = tongue roller, r = non-roller then calculate: TTbbRr x Ttbbrr  

Other forms of Inheritance Genetics is not simple. There are more complex gene interactions that occur other than what we term “simple Mendelian genetics”

Incomplete Dominance Incomplete dominance – when neither allele is dominant over the other and a blending of the traits is the result. Example – Hair Texture. Curly and Straight hair textures show incomplete dominance. CC = curly, SS = straight and CS = wavy.

Codominance Another example is codominance. Codominance is when both alleles are dominant and both traits show in the genotype. Blood Groups are an example. I is used for blood.

Blood Groups IA is for type A. IB is for type B i is for type 0 Persons with type A can be IA i or IA IA Persons with type B can be IB i or IB IB Persons with Type AB are IA IB Persons with 0 are ii.

Genetic Expression and the Environment Phenotypes are always affected by their environment. In buttercup (Ranunculus peltatus), leaves below water-level are finely divided and those above water-level are broad, floating, photosynthetic leaf-like leaves. Siamese cats are darker on their extremities, due to temperature effects on phenotypic expression. Expression of phenotype is a result of interaction between genes and environment.

Siamese cats and Himalayan rabbits both animals have dark colored fur on their extremities. This is caused by an allele that controls pigment production being able only to function at the lower temperatures of those extremities. Environment determines the phenotypic pattern of expression.

Polygenic Inheritance Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Polygenic traits are not expressed as absolute characters, as was the case with the pea plant traits.

Instead, polygenic traits are recognizable by their expression as a gradation of small differences (a continuous variation). The results form a bell shaped curve, with a mean value and extremes in either direction. Usually polygenic traits are distinguished by Traits are usually quantified by measurement rather than counting. Two or more gene pairs contribute to the phenotype. Phenotypic expression of polygenic traits varies over a wide range.

Linked Genes Genes close together on same chromosome are called linked genes Linked genes do not exhibit independent assortment and they move together during crossing over if they are very close together on the chromosome.

Sex Linked Genes Genes that are located on the x chromosome are sex linked. It is called an x-linked trait and is only carried on the x chromosome. Red-green colorblindness, hemophilia, and male pattern baldness are examples of x-linked traits.

Sex-Linked Genes