Genetics is the study of heredity.
Patterns of InheritanceThere are different patterns of inheritance that you will learn about, they are:
Mendelian Genetics: 1. Complete Dominance Non-Mendelian Genetics: 2. Incomplete Dominance (Intermediate Inheritance) 3. Co-dominance 4. Multiple Allele 5. Polygenetic Inheritance 6. Sex-Linked Inheritance - An organism's genotype is its genetic make up. Example: TT
- An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior, it reflects its overall genotype and unique environmental history. Example: Tall |
Tutorials/Videos
Worksheet/Assignments
Practice Worksheet 2_ Beyond the Basics
Practice EOC Questions
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Complete Dominance (Mendelian Genetics)
1. Organisms that reproduce sexually received genes from each parent. Those genes may be the same or may be different.
- such variations in inherited characters, are called alleles
2. There are alternative versions of genes
- Genes can be dominant, and are always expressed (represented with a capital letter) Ex. “A”
- Or recessive (represented with a lower-case letter) Ex. “a”
- The other allele, the recessive allele (p), has no noticeable effect on the organism’s appearance
- An organism that has two dominant or two recessive alleles for a trait is homozygous or purebred (two identical alleles of a particular gene)
- An organism is heterozygous or hybrid if it has two different alleles for a trait. (Pp)
Mendel's Laws of Inheritance describe the basic principles of inheritance. They are:
1. Law of Segregation
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2. Law of Independent Assortment
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3. Law of Dominance
Mendel's principle of dominance states that organisms have variations in the inheritable characters, which we call alleles. Some alleles are dominant, and some are recessive.
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Incomplete Dominance
- Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele in heterozygous individuals.
- The results is a third phenotype in which the expressed physical trait is a combination of the phenotype of both alleles. In the picture below you can see that neither red, nor white are observed in the F1 generation, but an intermediate form of both parent's colors (pink).
Codominance
- In codominance, both alleles are completely expressed when present in heterozygous individuals.
- They are clearly visible!
- The result is a combination of both physical traits simultaneously shown in the F1 generation.
- As shown here, both feather colors are present in the offspring of a cross between a black hen, and a white rooster.
- The roan coat color in cattle is another example of codominance. This means both alleles are clearly expressed.
Multiple Allele
Multiple alleles exist in a population when there are many variations of a gene present.
- Multiple Alleles for Coat Color in Rabbits This table show the phenotypes obtained by four alleles of the C gene for coat color in rabbits.
- Different combinations of two alleles give different coat colors and pigment distributions.
- They can be the same allele, which is called a homozygous genotype, or it can consist of alleles of different types, known as a heterozygous genotype.
Polygenic Inheritance
- Polygenic traits are determined by interactions among more than one gene.
- For example, kernel color in wheat has been proposed to be controlled by at least two genes. In the model shown by the Punnett Square, two genes each have a dominant allele (A or B) and a recessive allele (a or b).
- Based on the model shown by the Punnett Square below, the number of dominant alleles in the genotype is what determines kernel color.
- Human height is a quantitative trait.
- Many genes contribute to height, and the interactions between these genes are complex. But the environment also contributes to variation in height; for example, some people have better nutrition than others, and this can affect their growth.
Sex-Linked Inheritance
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within cell's mitochondria. The human genomes consists of both protein-coding DNA (genes) and noncoding DNA.
- Females are XX, they require two X chromosomes, males however, only have one X and one Y
- The X chromosome is quite large in comparison to the male chromosomes.
- There are many more genes in the X, that there are in the Y chromosome, see the picture below.
- This is particularly crucial in the transmission of sex linked genes. Patterns of inheritance previously studied are NOT OBSERVED IN THE TRANSMISSION OF SEX GENES therefore they are inherited differently between males and females.