Mendelian genetics, also known as classical genetics, refers to the principles of inheritance proposed by Gregor Mendel, an Austrian scientist, in the mid-19th century. His work laid the foundation for understanding how certain traits are passed from parents to offspring.
Mendel conducted experiments with pea plants and carefully observed their patterns of inheritance. He focused on specific traits, such as flower color (purple or white), seed texture (smooth or wrinkled), and plant height (tall or short). Through his experiments, Mendel formulated three key principles of inheritance:
1. Law of Segregation: Mendel proposed that an individual has two alleles (gene variants) for each trait, and these alleles segregate (separate) during the formation of reproductive cells (gametes, like sperm and eggs). Each gamete carries only one allele for each trait. When fertilization occurs, the offspring receive one allele from each parent, thus restoring the two-allele condition.
2. Law of Independent Assortment: Mendel discovered that different traits are inherited independently of each other. This means that the inheritance of one trait, like flower color, does not influence the inheritance of another trait, like seed texture. The alleles for different traits are randomly assorted and inherited separately.
3. Dominance and Recessiveness: Mendel observed that certain alleles are dominant, meaning their effect is seen when present in one or both copies, while others are recessive, and their effect is only observed when present in two copies. For example, if an individual has one dominant allele and one recessive allele for a trait, the dominant allele's characteristic will be expressed in the offspring.
These Mendelian principles provide a straightforward understanding of how specific traits are inherited in a predictable manner from one generation to the next. They laid the groundwork for modern genetics and helped scientists understand the basics of genetic inheritance, including the concept of genes, alleles, and the role of DNA in transmitting traits. Mendelian genetics continues to be an essential component of the broader field of genetics and heredity.
Mendel conducted experiments with pea plants and carefully observed their patterns of inheritance. He focused on specific traits, such as flower color (purple or white), seed texture (smooth or wrinkled), and plant height (tall or short). Through his experiments, Mendel formulated three key principles of inheritance:
1. Law of Segregation: Mendel proposed that an individual has two alleles (gene variants) for each trait, and these alleles segregate (separate) during the formation of reproductive cells (gametes, like sperm and eggs). Each gamete carries only one allele for each trait. When fertilization occurs, the offspring receive one allele from each parent, thus restoring the two-allele condition.
2. Law of Independent Assortment: Mendel discovered that different traits are inherited independently of each other. This means that the inheritance of one trait, like flower color, does not influence the inheritance of another trait, like seed texture. The alleles for different traits are randomly assorted and inherited separately.
3. Dominance and Recessiveness: Mendel observed that certain alleles are dominant, meaning their effect is seen when present in one or both copies, while others are recessive, and their effect is only observed when present in two copies. For example, if an individual has one dominant allele and one recessive allele for a trait, the dominant allele's characteristic will be expressed in the offspring.
These Mendelian principles provide a straightforward understanding of how specific traits are inherited in a predictable manner from one generation to the next. They laid the groundwork for modern genetics and helped scientists understand the basics of genetic inheritance, including the concept of genes, alleles, and the role of DNA in transmitting traits. Mendelian genetics continues to be an essential component of the broader field of genetics and heredity.
Dominant vs. Recessive
Mendel used letters like G and g to represent different alleles, like the allele for green or yellow pea pod color. The capital letter always stands for the dominant trait, and lowercase letter always stands for the recessive trait. When plants have two identical letters, like GG or gg, they are called "homozygous." If they have different letters, like Gg, they are called "heterozygous."
During reproduction, each parent gives allele to their offspring. So, if a GG plant and a gg plant have a baby, the baby gets one G from one parent and one g from the other parent, making it Gg. The combination of alleles results in the actual traits we observe. In our pea example, yellow color (G) is dominant to green color (g). In order to be green, a pea plant has to have two gg alleles, as green is the recessive alleles. A yellow pea plant could have GG alleles or Gg alleles.
Scientists call an organism's genetic letters its "genotype," and how it looks or its traits is its "phenotype." For example, even if two plants have the same yellow pod color (phenotype), they can have different letters (genotype), like GG or Gg. Understanding genotype and phenotype helps geneticists study how traits are passed down from parents to babies.
Mendel used letters like G and g to represent different alleles, like the allele for green or yellow pea pod color. The capital letter always stands for the dominant trait, and lowercase letter always stands for the recessive trait. When plants have two identical letters, like GG or gg, they are called "homozygous." If they have different letters, like Gg, they are called "heterozygous."
During reproduction, each parent gives allele to their offspring. So, if a GG plant and a gg plant have a baby, the baby gets one G from one parent and one g from the other parent, making it Gg. The combination of alleles results in the actual traits we observe. In our pea example, yellow color (G) is dominant to green color (g). In order to be green, a pea plant has to have two gg alleles, as green is the recessive alleles. A yellow pea plant could have GG alleles or Gg alleles.
Scientists call an organism's genetic letters its "genotype," and how it looks or its traits is its "phenotype." For example, even if two plants have the same yellow pod color (phenotype), they can have different letters (genotype), like GG or Gg. Understanding genotype and phenotype helps geneticists study how traits are passed down from parents to babies.
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Punnet Squares
A Punnett square is a simple and visual tool used by geneticists to predict the probability of different combinations of alleles and the resulting genotypes of offspring from a genetic cross between two parents.
A Punnett square is typically a square grid with rows and columns. The top of the square represents the alleles of one parent, and the left side represents the alleles of the other parent. The different combinations of alleles from the two parents are placed in the boxes within the grid to show all possible genotypes of the offspring.
The alleles are usually represented using letters, with uppercase letters indicating dominant alleles and lowercase letters indicating recessive alleles. For example, for a genetic cross involving the flower color of pea plants, where "P" represents the dominant allele for purple flowers and "p" represents the recessive allele for white flowers, a Punnett square would look like this:
A Punnett square is a simple and visual tool used by geneticists to predict the probability of different combinations of alleles and the resulting genotypes of offspring from a genetic cross between two parents.
A Punnett square is typically a square grid with rows and columns. The top of the square represents the alleles of one parent, and the left side represents the alleles of the other parent. The different combinations of alleles from the two parents are placed in the boxes within the grid to show all possible genotypes of the offspring.
The alleles are usually represented using letters, with uppercase letters indicating dominant alleles and lowercase letters indicating recessive alleles. For example, for a genetic cross involving the flower color of pea plants, where "P" represents the dominant allele for purple flowers and "p" represents the recessive allele for white flowers, a Punnett square would look like this:
Image Attribution: Madprime, CC0, via Wikimedia Commons
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In the punnet square to the left both parents have purple flower and are heterozygous (Bb).
It is possible for the offspring of these parents to get a B allele from each parent resulting in a BB - purple flowered - offspring. The are two possible ways in which the offspring would get B from one parent and a b from the other parent. This means there are two possibilities that result in a Bb - purple flowered - offspring. The final option is for the offspring to to get a b allele from each parent, resulting in a bb - which flowered - offspring. |
Dihybrid crosses will not be assessed in this course.
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