Notes 

Complex Patterns of Inheritance - Chapter 17 Notes

*Mendel’s laws of inheritance could not predict that most cats with calico coat are almost always female.

Group Question

Mendelian- Inheritance patterns in which a single gene affects a single trait and the alleles obey the law of segregation.

*Mendelian genetics formulated basic laws of inheritance and gave scientists a better understanding of sex, incomplete dominance and codominance.

*Most (almost all) traits are influenced by many genes

Epistasis-A type of gene interaction in which the alleles of one gene mask the effects of a dominant allele of another gene.

*Even plant height is determined by multiple genes, but the pea plants chosen by Mendel were identical in all but one locus.

Continuous Variation- Inheritance pattern in which the offspring display a continuous range of phenotypes.

             This pattern is produced by the additive interactions of several genes, along with environmental influences.

Linkage- Inheritance pattern involving two or more genes that are close together on the same chromosome.

Extranuclear inheritance- Inheritance pattern of genes found in the genomes of mitochondria or chloroplasts. Usually genes inherited by the mother.

X inactivation: phenomenon of female mammals in which the X chromosome is inactivated in every somatic cell, producing a mosaic phenotype.

Genomic imprinting: Inheritance pattern in which an allele from one parent is silenced in the somatic cells of the offspring, whereas the allele from the other parent is expressed.

Maternal effect: Inheritance pattern in which the genotype of the mother determines the phenotype of the offspring. This occurs because the mother provides the gene products from the maternal effects genes to developing egg cells.

Gene interaction:  situations in which a single character is controlled by two or more genes, each of which has two or more alleles.

Epistasis:  alleles of one gene mask the expression of the alleles of another gene.

*Bateson and Punnett studied the sweet pea in the early 1900s and deduced that two different genes were involved.

*Epistatic interactions often arise because two of more different proteins are involved in a single cellular function (p. 352 2nd column)

Discrete traits:  inheritance of characters with clearly defined phenotypic variants.

Quantitative traits: traits that show continuous variation over a range of phenotypes.

*Human traits like height, weight, skin color, heart size, and metabolic rate

*# of eggs a chicken lays, amount of milk a cow produces, and apples on an apple tree are also quantitative.

Agricultural research has produced the most research in this area.

Polygenic:  Several or many genes contribute to the outcome of the trait.

*Look at figure 17.2 on p.354

17.2 – Genes on the same chromosome: Linkage, Recombination, and Mapping

Linkage group: a group of genes that usually stay together during meiosis.

Linkage: When two or more genes are close to each other on a chromosome, they tend to transunit as a unit

*Linkage groups DO NOT follow the Law of Independent Assortment.

*Some sweet pea traits are linkage groups

*Thomas Morgan Hunt noticed similar ratios when studying the fruit fly. He proposed 3 ideas:

1) Genes located on the same chromosome are inherited together --- this violates independent assortment.

2) New combinations of alleles can be created due to crossing over in meiosis.

3) The further apart genes are on a chromosome, the more likely crossing over can/will occur.

*See Figure 17.4 p.357

Testcross=Crossing an individual with another individual with the homozygous recessive traits to determine the genotype of the first individual.

Nonrecombinants: Parental types ---- Have same traits as parents

Recombinants: the smaller number of offspring that have a combination of traits not found in the parental generation.

Genetic mapping: the study of the arrangement of genes in a species’ genome.

Genetic map: the linear order of genes that are linked to each other along the same chromosome is shown in a chart.

*Alfred Sturtevant made the first genetic map. The map showed 5 genes on the fruit fly.

*He was a student in Morgan’s laboratory.

Recombination frequency: the frequency of crossing over between two genes.

Map distance: the number of recombinants divided by the total number of offspring times 100.

Map units: the units of distance. CentiMorgans are equal to 1% combination frequency

*Ex. 270 – out of 1000 recombinants is 27% recombination frequency or mu apart.

17.3 – Extra Nuclear Inheritance: Organelle Genomes

Extranuclear Inheritance (or Cytoplasmic inheritance): transmission of genes located outside of the nucleus.

*Some genes are not located in chromosomes in the cell nucleus.

*These genes do not segregate in the same way

*Mitochondrial genome of many mammalian species contain a total of 37 genes.

**24 genes encode tRNAs and rRNAs and 13 genes encode proteins that are involved in oxidative phosphorylation.

*chloroplast’s genome typically contain 110-120 genes. Most of the vital to photosynthesis

Maternal inheritance: pigmentation that depends solely on the female parent.

Biparental inheritance:  both the pollen and egg contribute chloroplasts to the offspring.

Parental inheritance:  only the pollen contributes chloroplasts to the offspring.

17.4 –X Inactivation, Genomic Imprinting, and Maternal Effect

Epigenetic inheritance: modification of a gene or chromosome during egg formation, sperm formation, or early stages of embryo growth alters gene expression in a way that is fixed during an individual’s life-time.

Maternal effect (p.361): the genotype of the mother directly determines the phenotype of her offspring.

X-inactivation (p.362): One X-chromosome in the somatic cell of female is inactivated.

Barr body: the highly condensed structure that were found only in female cats

*1949 Murray Barr and Ewart Bertram identified a highly condensed structure in the cells of female cats, but not male cats.

*1960 Susumu Ohno correctly proposed that a Barr body is a highly condensed X-chromosome.

*Mary Lyon proposed X inactivation in 1961 based on the Barr body and calico cats

Mosaics: heterozygotes that are composed of two type of cells

Dosage compensation: a process that equalizes the expression of X-linked genes in male and female mammals.

X inactivation center: Xic: A short region on the X chromosome known to play a critical role in X inactivation.

Eeva Therman and Klaus Patau determined X activation is accomplished by counting the number of Xics in activating all X chromosomes except for one.

Having two active X chromosomes is a lethal condition for a human female embryo.

*See chart on p. 363

A specific gene within the X inactivation center is required for compaction of the X chromosome into a Barr body.

*The gene Xist(X inactive specific transcript) was discovered in 1991.

The Xist gene product is a long RNA molecule that does not encode a protein.  The role of the Xist RNA is to coat one of the two X chromosomes during the process of X activation.   This promotes compaction of the chromosome into a Barr body.  The Barr body replicates during cell division.

Genomic imprinting: refers to an inheritance pattern in which a segment of DNA is imprinted or marked so that gene expression occurs only from the genetic material inherited from one parent.

Maternal effect genes: only the genotype of the mother effect the offspring (the father does NOT effect the offspring).