Study Guide Reviews:
- Basic Concepts of Inheritance
- Alleles, Phenotype, Genotype
- Punnett Square and Dihybrid Cross
- Non-Mendelian Inheritance
- Mendel's Law of Dominance
- Mendel's Law of Segregation
- Mendel's Law of Independent Assortment
ETC.
Basic Concepts of Inheritance
Inheritance is the process by which traits are passed from parents to their offspring.
Traits can be either dominant or recessive.
Mendel’s law of inheritance states that each trait is determined by two alleles.
Alleles are alternative forms of a gene, where one is dominant and the other is recessive.
The phenotype of the dominant allele is the one that is expressed.
Alleles and Phenotype
- Each parent contributes one allele to their offspring, resulting in a combination of
observable characteristics or traits known as the phenotype.
- For example, if one parent has the allele for blue eyes and the other has the allele for
brown eyes, the offspring will have the alleles for both eye colors, with the phenotype
being brown eyes due to the dominance of the brown allele.
- If both parents have alleles for blue eyes, their offspring has the chance of having blue
eyes as their phenotype.
Genotype refers to the combination of two alleles, and it can be either heterozygous (two
different alleles) or homozygous (two identical alleles) for a trait.
Punnett Square and Dihybrid Cross
When using a Punnett square to multiply alleles, the dominant letter is used first, with dominant
letters being capitalized.
- In a dihybrid cross, the expected ratio is 9:3:3:1.
- Exceptions to Mendel’s law of inheritance include incomplete dominance and
codominance.
Non-Mendelian Inheritance
Incomplete dominance occurs when the phenotype of the offspring is a blend of the phenotypes
of the parents, such as when a red flower is crossed with a white flower resulting in pink
offspring.
Codominance occurs when the phenotype of the offspring is a combination of the phenotypes
of the parents, for example, when a black chicken is crossed with a white chicken resulting in
black and white offspring.
Linkage refers to when two genes are located close to each other on the same chromosome
and are inherited together, often observed in sex-linked inheritance.
Sex-linked inheritance involves a gene located on the X orY chromosomes, with color
blindness being a common example caused by a gene located on the X chromosome
, Mendel's Law of Dominance
Mendel’s Law of Dominance states that ‘In crossing between homozygous organisms for
contrasting characters of a pair, only one character of the pair appears in the first generation.’
The law of dominance is the first law of heredity proposed from the works of Mendel. The law
explains that all characters in an individual are controlled by distinct units called factors that
occur in pairs.
- The pair can be homozygous or heterozygous, and in the case of heterozygous pairs,
one of the factors dominates the other.
- The character that dominates is called the dominant character, and the one that
remains unexpressed is the recessive character.
- The recessive character, even though latent, is transmitted to the offspring in the same
way as the dominant character.
- The recessive character is only expressed when the offspring has two copies of the
same allele resulting in a homozygous individual.
- The two alleles responsible for a character are brought together during fertilization,
where one of the alleles comes from the maternal gamete and the other from the
parental gamete.
- The concept of dominance is strictly only used for genotypic characters and does not
represent the phenotype of the individual.
- With new experiments on genetics, many researchers believe that the law of dominance
doesn’t always hold true and that other patterns of inheritance also exist.
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