The laws of Heredity. Allele: is the copy (or a version) of the gene that control the same characteristics.

Transcription

1 The laws of Heredity 1. Definition: Heredity: The passing of traits from parents to their offspring by means of the genes from the parents. Gene: Part or portion of a chromosome that carries genetic information responsible for certain trait (inherit from their parent). Each individual has two copies of the gene (paternal and maternal). The two copies of the gene occupy the same location on the homologous pair of chromosomes. Allele: is the copy (or a version) of the gene that control the same characteristics. Allele Dominant Recessive Recessive allele: is the allele that is masked by another dominant allele, it is only expressed in homozygote state. Dominate allele: is the allele that makes the presence of the other alleles, it can be expressed in one or two alleles.

2 Codominance: when both alleles are expressed at the same time A is codominant with B-neither covers the other both are expressed independently. A is dominant over O- Only A is expressed if A and O are present. B is dominant over O- Only B is expressed if B and O are present. Blood Type Genotype Phenotype A AA or AO Blood type A B BB or BO Blood type B AB AB Blood type AB O OO Blood type O Phenotype: is the observable or visible characteristic it is the expression of the genotype. Genotype: is the genetic makeup of an individual composed of all the genes inherited from the parents. F1. Homozygous (pure): when the 2 alleles of genotype are the same. Ex: AA. Heterozygous (hybrid): when the 2 alleles of genotype are different. Ex: Ab. 2. Mendel s law: A- Exercise: Green peas were crossed with yellow peas (parents). Only green peas resulted in 1- Indicate the dominant allele. Justify the answer. Green color dominates the yellow color peas. 2- Designate by symbols the corresponding alleles. Green is the dominate gene and is symbolized as G. Yellow is the recessive gene and is symbolized as Y. 3- Write the genotypes of parents and the first generation. *Note: Both parents were pure (homozygous) since F1 were all the same.

3 Genotype of green: GG Genotype of yellow: yy Genotype of F 1 : Gy 4- Make a factorial analysis. Factorial analysis: Phenotype: Green x yellow Genotype: GG x yy Gametes:100% G 100% y Or 1 st law: There is uniformity in F1, uniformity in F1 means purity of parents. B- Self Cross: if we cross F1 x F1 : Phenotype: Green x Green Genotype: Gy x Gy

4 C- Test/Back Cross: Crossing heterozygous F1 by homozygous recessive: Phenotype: F 1 (Green) x yellow Genotype: Gy x yy Table of cross:

5 III. Incomplete Dominance & Codominance: If both alleles are dominate: A- Incomplete dominance if an intermediate trait appeared in F1. B- Codominance if both traits were expressed in F1. A- Exercise: Red flowers crossed with white flowers. Only pink flowers appeared in F1. - Both parents are pure (gave pink only). -Alleles (red & white) are incompletely dominant (intermediate new trait). R: symbol of the red allele which is non- dominant. E: symbol of the white allele which is non- dominant. Phenotype: Red x White Genotype: RR x WW Table of cross: B- Self Cross: Phenotype: Pink x pink Genotype: RW x RW

6 Table of cross: V. Pedigree (Family Tree): Pedigree: the history of the family members in a person's or animal's past especially when it is good or impressive. A. Symbols : - Square : male Circle : female - Line joining a square and a circle : mating (cross-marriage) - Colored square or circle : diseased character B. Mode of transmission of the disease: Mode of transmission of the disease Recessive (only expressed in homozygous state) Dominant (Expressed in homozygous or heterozygous state)

7 1) Recessive Disease (autosomal): The disease is recessive when two normal parents gave birth to an affected child, so the allele that is hidden in parents will appear in children (masked). This disease is recessive because (II-1) is affected (diseased) while his parents (I-1) and (I-2) are normal. The allele of the disease is hidden in parents. Let N be the symbol of the normal allele which is dominant. Let a be the symbol of the affected (abnormal) allele which is recessive. Genotypes of I-1 & I-2 : Na (normal but having affected child, carriers of the disease) II-2: aa (disease is recessive, only expressed in homozygous state) II-1 & II-3 : can be either Na or NN 2) Dominant Disease (autosomal): a- For every affected child, there exists at least one affected parent. b- The disease is found in all generations (in most cases). c- Affected parents can have normal children. In the this pedigree, the disease is dominant, since (I-1) and (I-2) are diseased parents, but gave birth to normal children (II-2) and (II- 5). Hence, the normal allele is masked in parents (recessive), and the disease is dominant. Let D be the symbol of the disease dominant allele. Let n be the symbol of the normal recessive allele. d- Genotypes of I-1 & I-2: Dn (affected but gave birth to a normal child, so normal allele is masked by the affected one). II-1 & II-2 & II-4: DD or Dn (affected and the dominant allele is dominant, so it can be expressed in one or two alleles). II-3: nn (recessive normal allele, can be only expressed in two alleles)