ZOOHCC - 501: Principles of Genetics (Theory)
Unit 1: Mendelian Genetics and its Extension
Unit 1: Mendelian Genetics and its Extension
Principles of inheritance
Mendelian inheritance refers to the pattern of inheritance of genetic traits that follow specific rules first described by Gregor Mendel, an Austrian monk and scientist, in the 19th century. These rules describe how certain traits are passed down from one generation to the next through the inheritance of genes.
The principles of Mendelian inheritance include:
Law of Segregation: This states that an individual has two copies of each gene, and these copies segregate during the formation of gametes (eggs and sperm) so that each gamete receives only one copy.
Law of Independent Assortment: This states that the inheritance of one gene does not influence the inheritance of another gene, and genes assort independently during gamete formation.
Dominance and Recessiveness: This refers to the fact that some alleles (versions of a gene) are dominant over others, meaning that they are expressed in the phenotype (physical appearance), while recessive alleles are only expressed in the phenotype when two copies are present.
These principles of Mendelian inheritance provide a foundation for understanding how genetic traits are passed down from one generation to the next and can be used to predict the probability of certain traits appearing in offspring.
Law of Dominance first law
The Law of Dominance is one of the three fundamental principles of Mendelian inheritance. It states that when two different alleles (variations of a gene) are present in an individual, one of them (the dominant allele) will be expressed in the phenotype (physical appearance), while the other (the recessive allele) will be masked or not expressed.
For example, let's consider the gene for flower color in pea plants. The gene has two alleles: one for purple flowers (P) and one for white flowers (p). When a pea plant has two copies of the dominant allele (PP), it will have purple flowers. If it has one copy of the dominant allele and one copy of the recessive allele (Pp), it will still have purple flowers because the dominant allele masks the recessive allele. Only when a pea plant has two copies of the recessive allele (pp) will it have white flowers, because there is no dominant allele present to mask the recessive allele.
This pattern of inheritance is seen in many different traits in organisms, including human traits like eye color and blood type. The Law of Dominance helps explain why some traits are more common than others and how genetic variation is maintained in populations.
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The law of dominance can also be illustrated with the example of plant height in pea plants.
In pea plants, plant height is determined by a single gene with two possible alleles: the dominant allele (T) for tall plants and the recessive allele (t) for dwarf plants. When a pea plant inherits two dominant alleles (TT) or one dominant and one recessive allele (Tt), it will be tall because the dominant allele is expressed. Only when a pea plant inherits two recessive alleles (tt) will it be dwarf, because there is no dominant allele to mask the expression of the recessive allele.
For example, if a pea plant with a heterozygous genotype (Tt) for plant height is crossed with another pea plant with the same genotype, the Punnett square for the cross predicts that 25% of the offspring will have the homozygous dominant genotype (TT) and be tall, 50% of the offspring will have the heterozygous genotype (Tt) and also be tall, and 25% of the offspring will have the homozygous recessive genotype (tt) and be dwarf.
As you can see, all of the F1 offspring are tall because they inherit one dominant allele (T) from the tall parent. However, they are all heterozygous (Tt) because they also inherit one recessive allele (t) from the dwarf parent.
Law of Segregation
The Law of Segregation is one of the three fundamental principles of Mendelian inheritance. It states that when gametes (eggs and sperm) are formed during sexual reproduction, the two alleles (versions of a gene) that an individual possesses for a particular trait separate from each other and only one allele is passed down to each gamete.
For example, let's consider the gene for flower color in pea plants. The gene has two alleles: one for yellow flowers (Y) and one for green flowers (y). When a pea plant with genotype Yy produces gametes, the two alleles separate from each other during the formation of eggs and sperm, so that each gamete receives only one allele. Thus, half of the gametes will contain the Y allele and the other half will contain the y allele. When the gametes combine during fertilization, the resulting offspring will inherit one allele from each parent, and the genotype of the offspring will be determined by which alleles they receive.
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The Law of Segregation helps explain how genetic variation is passed down from generation to generation and how different combinations of alleles can lead to different phenotypes (physical appearances) in offspring. It is an important principle of genetics that has been verified by countless experiments and observations.
Law of independent assortment
The law of independent assortment is a principle of genetics that states that the inheritance of one trait does not affect the inheritance of another trait. This means that when two or more traits are inherited, they are inherited independently of each other.
This law was first proposed by Gregor Mendel, the father of modern genetics, in the mid-1800s. Mendel conducted experiments with pea plants and found that the inheritance of one trait, such as seed color, was independent of the inheritance of another trait, such as seed shape.
Let's look at a concrete example of the law of independent assortment. Imagine that we cross two pure-breeding pea plants: one with yellow, round seeds (YYRR) and one with green, wrinkled seeds (yyrr). Because each parent is homozygous, the law of segregation tells us that the gametes made by the wrinkled, green plant all are ry, and the gametes made by the round, yellow plant are all RY. That gives us F1 offspring that are all RrYy.
The allele specifying yellow seed color is dominant to the allele specifying green seed color, and the allele specifying round shape is dominant to the allele specifying wrinkled shape, as shown by the capital and lower-case letters. This means that the F1 plants are all yellow and round. Because they are heterozygous for two genes, the F1 plants are called dihybrids (di- = two, -hybrid = heterozygous).
A cross between two dihybrids (or, equivalently, self-fertilization of a dihybrid) is known as a dihybrid cross. When Mendel did this cross and looked at the offspring, he found that there were four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. These phenotypic categories (categories defined by observable traits) appeared in a ratio of approximately 9:3:3:1.
This ratio was the key clue that led Mendel to the law of independent assortment. That's because a 9:3:3:1 ratio is exactly what we'd expect to see if the F1 plant made four types of gametes (sperm and eggs) with equal frequency: YR, Yr, yR, and yr. In other words, this is the result we'd predict if each gamete randomly got a Y or y allele, and, in a separate process, also randomly got an R or r allele (making four equally probable combinations).
We can confirm the link between the four types of gametes and the 9:3:3:1 ratio using the Punnett square above. To make the square, we first put the four equally probable gamete types along each axis. Then, we join gametes on the axes in the boxes of the chart, representing fertilization events. The 16 equal-probability fertilization events that can occur among the gametes are shown in the 16 boxes. The offspring genotypes in the boxes correspond to a 9:3:3:1 ratio of phenotypes, just as Mendel observed.
The law of independent assortment applies to genes located on different chromosomes or to genes located far apart on the same chromosome. However, genes located close together on the same chromosome are more likely to be inherited together, a phenomenon known as genetic linkage.
The law of independent assortment has important implications for understanding the inheritance of genetic traits and for predicting the outcomes of genetic crosses. It allows geneticists to make predictions about the likelihood of particular traits appearing in offspring, based on the known genetic makeup of the parents.
Mendelian genetics and human disease | Genetic counseling and inheritance patterns | Genetic testing and Mendelian disorders
