General Biology/Genetics/Inheritance

General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material

---

Charles Darwin, for all he contributed to the science of biology, never knew about the mechanism by which living things inherit traits from previous generations, or how new traits arise.

As any schoolchild can tell you, this mechanism of interitance has since been found to be DNA, or deoxyribonucleic acid. DNA allows for stable inheritance of traits: the code in each strand of DNA is replicated precisely through the pairing of basic units along each strand. The error rate in this replication is amazingly low; not even one base pair in a million matches out of sequence.

However, when even one base pair is added to a new strain of DNA in an order differently than in the parent chain, it can be the basis of a mutation. These changes in DNA sequences are the microscopic origin of changes in traits of all studied living things. Even the smallest difference in a strand of DNA can result in a change in traits that can cost the life of the organism. Mutations can produce proteins with a new or altered function. In humans, the example of Sickle cell anemia is commonly given as its origin is a difference of only one base pair in a section of DNA that encodes red blood cells.

Individual sequences of DNA that encode for specific proteins are called genes and are the units of heredity. Each one has a set nucleotide, and together all of the genes (and some sequence of DNA that apparently do not code for any biologically important functions) together make up the entire chromosome

Mendel

• Discovered principle of genetic segregation via numerous experiments utilizing pea plants
• Inferred the existence of genes through segregation of phenotypes
• Used quantitative methods: counted; ratios
• Work is model of scientific method
• In particular, observed the F2 progeny, which lead to the discovery of dominant and recessive traits
• Published work in 1866, went unnoticed
• In 1900 his scientific paper was “rediscovered”
• Mendel is acknowledged as founder of Genetics
  • still used alphabet letters to designate genes
  • still refer to dominant and recessive genes
  • still refer to segregation of alleles in meiosis
  • principle of segregation applies to all sexually reproducing organisms; Mendel’s results were immediately applied to humans in 1900

Mendel’s experiments

• 1856, began experiments with the garden pea, Pisum sativum
• 1865, presented results to the Bruno natural history society, which he helped found
• 1866, published his results in proceedings of the society
• Naegeli encouraged Mendel to reproduce results in another species, which failed because the species did not undergo true fertilization
• discrete traits in Pisum sativum
  • pure-breeding lines
  • dominant/recessive alleles
• alleles are two alternate versions of a gene
  • gametes contain hybridized chromosomes that are formed during meiosis
  • homozygous has two of the same allele
  • heterozygous has two different alleles
• reciprocal F1 crosses (all exhibiting dominant phenotypes); F2; F3
• counted offspring, noted ratios
• inferred genotypes from phenotypes
• tested hypotheses with testcrosses
• attempted to repeat with another species

Mendel’s seven pairs of traits

1. Seed form (round or wrinkled)
2. Cotyledon color (green or yellow)
3. Seed coat color (white or colored)
4. Pod form (inflated or constricted)
5. Pod color (green or yellow)
6. Flower position (axial or terminal)
7. Plant heights (tall or short)

Locus

• The location of a specific gene within a chromosome

Modern Y chromosome

Y-chromosome is the most evolved chromosome. Generally it is thought that if Y- chromosome is present in an individual then he will be male. But if mutation occurs at sex determining region or zinc factor then it will not code for testis determining factor, and results in normal female. This type of female's frequency is 1/250000.

Chromosome phenomena

• X-chromosome inactivation
  • Barr bodies
• Nondisjunction: failure of chromosome segregation at meiosis or mitosis
  • Results in 2N ± 1 chromosome number
• Trisomy 2N + 1
  • Usually lethal. Trisomy 21 (Down) exception
• Monosomy 2N ** 1
  • Lethal except XO
  • Usually maternal origin in humans

X-chromosome inactivation

In females, one X-chromosone is randomly switched off forming a Barr body.

Barr body

Dense region in the nucleus formed by the inactive X-chromosome.

Human genetic disorders

Down's Syndrome (Mongolism)

Down's Syndrome is usually produced by the nondisjunction of chromosome 21 during oogenesis and sometimes during spermatogenesis. The individual suffering from this type of syndrome has 47 chromosomes instead of the normal 46. The extra chromosome is not a sex chromosome but an autosome.

Most cases of mongolism were found to occur in children born by women in their forties. The affected children, called mongoloids, show mental retardation and have a shorter life expectancy. Their most prominent feature is the Mongolian folds in their eyes; hence, the term mongolism.

Klinefelter's Syndrome

When an XY-bearing sperm unites with an X-bearing egg, the resulting condition is called Klinefelter's Syndrome, or sexually undeveloped male. Individuals having the syndrome show the following characteristics:

• testes are small
• sperms are never produced
• breasts are enlarged
• body hair is sparse
• individuals are mentally defective

The same abnormal meiotic division may occur in females. They produce eggs with XX or no sex chromosomes. Such egg, when fertilized by a Y-bearing sperm, will not develop (YO). This is because YO is lethal—it wil cause death to the offspring.