Linkage and pedigree analysis
- 1. LINKAGE & PEDIGREE ANALYSIS Presented By: Collins Onteri Ongeri MSc. Biotechnology 191127202007 Parul University
- 2. LINKAGE • Genes which determine the characters of an individual are carried in the chromosomes. • An individual usually has many genes for the determination of various different characters. • As there are more genes than chromosomes, it can be expected that each chromosome contains more than one gene. • For instance, we humans have roughly 19, 000 genes on 23 chromosomes. • Fruit fly has 13,000 genes on four chromosomes. INTRODUCTION
- 3. • The genes for different characters may either be situated in the same or in different chromosomes. • When the genes are situated in different chromosomes, the characters they control appear in the next generation either together or apart. • They usually assort independently according to Mendel’s law of independent assortment. • But if the genes are located on the same chromosome and are fairly close to each other, they tend to be inherited together. • This type of coexistence of two or more genes in the same chromosome is known as linkage.
- 4. • When genes are linked, genetic crosses involving those genes will lead to ratios of gametes (egg and sperm) and offspring types that are not what we'd predict from Mendel's law of independent assortment.
- 7. Chromosomal Theory Of Linkage • Morgan and Castle formulated the chromosomal theory of linkage which is as follows: a. The genes that show the phenomenon of linkage are situated in the same chromosome and these linked genes usually remained bound by the same chromosomal material so that they cannot be separated during inheritance. b. The distance between the linked genes determines the strength of linkage. The closely located genes show strong linkage than the widely located genes. c. Genes are usually located in linear fashion in the chromosomes.
- 8. KINDS OF LINKAGE • Linkage is generally classified on the basis of: Crossing over Chromosomes involved
- 9. Linkage Based on Crossing Over a. Complete linkage • Complete linkage is a phenomenon in which parental combinations of characters appear together in two or more generations in a continuous and regular fashion. • The genes are completely linked and inherited as a set and no recombination is there. • Example: The genes for bent wings (bt) and shaven bristles (svn) on the fourth chromosome mutant of Drosophila exhibit complete linkage.
- 10. b. Incomplete linkage • Incomplete linkage is also called partial linkage. • The genes are not always together because homologous non-sister chromatids may exchange segments of varying length with one another during meiotic prophase.
- 11. Linkage Based on Chromosomes a. Autosomal linkage • It refers to linkage of those genes that are located in autosomes. b. Sex linkage • It refers to linkage of genes that are located in the sex chromosomes.
- 12. Linkage Groups • All the linked genes of a chromosome form a linkage group. • Because all the genes of a chromosome have their identical genes on the homologous chromosomes. • Therefore, linkage groups of a homologous pair of chromosome is considered as one. • The number of linkage groups of a species corresponds with the haploid chromosome number of that species. • Example: Human beings have 23 pairs of chromosomes and 23 linkage groups. • Maize has 10 pairs of chromosomes and 10 linkage groups.
- 13. PEDIGREE ANALYSIS INTRODUCTION • Pedigrees are used to analyze the pattern of inheritance of a particular trait throughout a family. • Pedigrees show the presence or absence of a trait as it relates to the relationship among parents, offspring, and siblings. • Geneticists use a standardized set of symbols to represent an individual’s sex, family relationships and phenotype. • These diagrams are used to determine the mode of inheritance of a particular disease or trait, and to predict the probability of its appearance among offspring. • Pedigree analysis is therefore an important tool in both basic research and genetic counseling.
- 15. • By analyzing a pedigree, we can determine genotypes, identify phenotypes, and predict how a trait will be passed on in the future. • The information from a pedigree makes it possible to determine how certain alleles are inherited: whether they are dominant, recessive, autosomal, or sex-linked. • To start reading a pedigree: 1. Determine whether the trait is dominant or recessive. If the trait is dominant, one of the parents must have the trait. • Dominant traits will not skip a generation. • If the trait is recessive, neither parent is required to have the trait since they can be heterozygous.
- 16. 2. Determine if the chart shows an autosomal or sex-linked (usually X-linked) trait. • For example, in X-linked recessive traits, males are much more commonly affected than females. • In autosomal traits, both males and females are equally likely to be affected (usually in equal proportions).
- 17. CATEGORIES OF INHERITANCE AUTOSOMAL • Autosomal means that the disorder in inherited on chromosomes 1-22. • There is autosomal recessive and autosomal dominant. Autosomal recessive • In autosomal recessive, the traits are rare in the pedigree. • Most often, the traits are not inherited from generation to generation in a continuous manner. • They tend to skip some generations. • The traits are hidden in heterozygous carriers. • The traits affect male and female equally.
- 18. • The possible examples include: cystic fibrosis, sickle cell anemia, phenylketonuria and Tay-Sachs disease. Autosomal dominant • The trait is common is common in the pedigree. • The trait is found in every generation. • The affected individuals usually transmit the trait to about ½ of the children regardless of sex. • There are few examples of autosomal dominant diseases. • Examples include: Huntington’s disease and achondroplasia.
- 20. SEX-LINKED INHERITANCE • Sex-linked means that the trait is inherited on either the X or Y chromosome. • Similarly, we have sex-linked recessive and sex-linked dominant. • The traits that are inherited on the X chromosome are termed as X-linked. • X-linked genes have distinctive inheritance patterns because they are present in different numbers in females (XX) and males (XY). • X-linked human genetic disorders are much more common in males than in females due to the X-linked inheritance pattern. • And those inherited on the Y chromosome are termed as Y-linked.
- 21. CHARACTERISTICS OF SEX-LINKED INHERITANCE • The frequency of individuals showing a recessive sex-linked trait is markedly higher in heterogametic sex than that in homogametic sex. • The gene governing the sex linked traits are not transmitted from male parent directly to their male progeny. • Inheritance of sex-linked traits does not follow normal segregation patterns. In humans and drosophila, male transmits its sex-linked genes to all its daughters. These daughters then pass the trait to males • Thus sex linked get inherited in a pattern called crisscross pattern of inheritance.
- 22. X-LINKED INHERITANCE • X-linked diseases are inherited on the X chromosome. • X-linked diseases occur more frequently in males because they only have on X chromosome. • X-linked inheritance maybe dominant or recessive. X-linked dominant • X-linked genes show dominance only in homogametic sex (female) because it can carry two alleles at the sex-linked locus. • So females can be homozygous or heterozygous. • If the mother carries the trait, then: 50% of sons or/and daughters will be affected 50% of sons or/and daughters will be normal
- 24. • If the father carries the gene, then: 100% of the daughters will have the disorder 0% of the sons will have the disorder • If both parents have the disorder, then: 100% of daughters will have the disorder 50% of sons will have the disorder 50% of sons will be normal • Examples of X-linked dominant disorders include Rett syndrome X-linked lissencephaly Double-cortex syndrome Incontinentia pigmenti type 1
- 26. X-linked recessive • In X-linked recessive, the male can display the trait by having only one copy allele but in female it displays when both recessive alleles are present. • Examples: color blindness and haemophilia. • Males have only one X chromosome, and a single recessive allele on that X chromosome will act as pseudo dominance and cause the disease. • Females have two X chromosomes, so two copies of the recessive allele are required for the disease to express in female. • Males never pass the disease to their sons because there is no male-to-male transmission of the X-chromosome.
- 27. • Male pass the defective X-chromosome to all their daughters. • Female carriers have only one copy of the gene and usually don’t express the phenotype. Y-LINKED INHERITANCE • The Y-chromosome is small and doesn’t contain numerous genes. • Y-linked disorders are very rare. • The genes present on the Y chromosome are known as holandric genes. • The best example of a holandric condition is the presence of excessive hair on ears in man. • Y-linked genes would be transmitted directly from father to son and never appears in female.
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