Sex-linked Traits and Human Chromosomes

Sex Linkage and Recombination

• Recombination occurs when homologous chromosomes exchange genes
• Replication is the process of producing another copy of chromosomes

Sex Linkage

• The 1st to 22nd pairs of human chromosomes are called autosomes
• The 23rd pair is termed the sex chromosome
• Genes located on either sex chromosome are considered sex-linked

Human Karyotype

• A trait linked to the X chromosome is an X-linked trait
• A trait linked to the Y chromosome is a Y-linked trait

X-linked Traits

• X-linked traits are more common in males than in females
• Even if the mother is a carrier and the father is normal, there's still a chance of an offspring inheriting the X-linked trait
• Males generally have a 50% chance of expressing the trait (50% normal, 50% with the X-linked trait)
• Females have a 33.3% chance of each of the following: being normal, being a carrier, or manifesting the X-linked trait

Color Blindness

• Colorblindness affects the ability to distinguish certain colors
• The Ishihara chart is used to test for color blindness

Hemophilia

• Hemophilia is a rare, inherited bleeding disorder
• People with hemophilia bleed longer after injuries compared to normal individuals

Y-Linked Trait

• The Y-linked trait is only common in males that have a Y chromosome
• An example is hypertrichosis pinnae auris, characterized by hairy ears

Sex-Related Inheritance

• In contrast to Mendelian inheritance, a female with the heterozygous genotype for an X-linked trait will be a carrier

Sex-Influenced Trait

• Sex-influenced traits are controlled by autosomal genes
• These genes are present in both sexes, but express differently
• Human pattern baldness is a sex-influenced characteristic

Sex-Limited Trait

• Sex-limited traits are controlled by autosomal genes
• These genes are found in both sexes, but only one sex can express them
• Lactation is a female-limited trait

Sex Linkage Variation

• Sex linkage is so named because phenotypic manifestation and inheritance patterns vary between males and females
• Genes that travel with either sex chromosome are therefore sex-linked

Gene Interaction Review

• Alleles of a gene separate during gamete formation (Law of Segregation)
• Alleles from different loci assort independently (Law of Independent Assortment)
• In heterozygous individuals, the dominant allele masks the expression of the recessive allele completely (Principle of Dominance)
• Offspring ratios for the F2 generation are 3:1 (monohybrid) and 9:3:3:1 (dihybrid)
• One gene does not interact with another to control a trait

Extensions of Mendelian Inheritance

• Allele expression does not always follow simple dominance
• There may be a greater number of allelic variations for a single gene
• A single trait may have more phenotypic variations
• Results can give offspring ratios different from 3:1 and 9:3:3:1

Incomplete Dominance

• Two-allele system
• Blending of homozygous phenotypes
• The heterozygote has an intermediate phenotype
• There are at most three phenotypic classes

Codominance

• Simultaneous expression of two alleles
• Two-allele system
• No blending of homozygous phenotypes
• The heterozygote shows both alleles
• There are at most three phenotypic classes

MN Blood Group

• Blood type is determined by antigens
• Antigens are membrane glycoproteins
• Antigens are detected by the immune system
• There are two antigens: M and N
• Presence of an antigen is tested for through agglutination

Multiple Alleles

• Determined by the presence of antigens A and B on the surface of red blood cells
• Three different alleles: IA (A antigen), IB (B antigen), and i (no antigen)
• IA and IB are codominant
• Both are dominant over i
• Must be determined before any procedure related to blood transfusions

Gene Interaction

• Interaction between two different gene loci
• The interaction controls the expression of only one biological trait
• Epistasis: one gene masks the expression of another
• The epistatic gene masks the hypostatic gene
• The relationship is similar to dominant and recessive alleles

Allele Interaction

• The recessive allele of one gene masks the expression of the allele of another gene
• The dominant allele of one gene masks the expression of the allele of another gene

Classical and Modern Breeding

• Breeding is the controlled sexual reproduction of two parent organisms

Objectives of Plant Breeding

• Increase vegetative parts and grain yield
• Improved processing quality
• Improved nutrient quantity
• Stress resistance (salinity, drought, frost)
• Diseases resistance (bacterial, viral, fungal)
• Pest/Insect resistance

Objectives of Animal Breeding

• Captive breeding of the Philippine crocodile (Crocodylus mindorensis), a critically endangered freshwater crocodile once believed extinct
• A captive breeding facility was established at Silliman University of Negros Oriental to combat this
• Captive breeding of the Philippine eagle (Pithecophaga jefferyi), a critically endangered eagle species threatened by habitat loss
• A captive breeding facility was established at the Philippine Eagle Center in Davao City

Historical Progress of Plant Breeding

• 9000 BC: First archeological evidence of crop domestication was found at Tigris River
• 1694: Rudolph Camerarius proposed new plant types through crossing
• 1719: Thomas Fairchild conducted the first reported hybridization (sweet William and carnation)
• 1766: Joseph Koelreuter produced the first hybrid tobacco in Germany
• 1866: Gregor Mendel laid foundation to heredity of traits
• 1926: The Pioneer Company was a pioneer in hybridization

Historical Progress of Animal Breeding

• 1760s: Robert Bakewell introduced performance records for livestock
• 1910: Thomas Hunt Morgan received a Nobel Prize for breeding fruit flies
• 1918: Ronald Fisher applied statistics in mouse and livestock breeding
• 1937: Jay Lush recommended using genetic data rather than subjective animal appearance
• 1840s: Lanoy Nelson Hazel proposed the selection index theory

Plant and Animal Breeding

• Plant breeding aims to improve quality and quantity of crops
• Animal breeding aims to improve livestock quality and quantity
• Animal breeding may involve inbreeding or crossbreeding

Classical Plant Breeding Techniques

• Selective breeding
• Crossbreeding

Pure-Line Selection

• Aims to establish a breed that is homozygous for particular traits, achieved with self-pollination

Mass Selection

• Aims to improve qualities by selecting good-quality offspring and open pollination

Clonal Selection

• Is applicable for good-quality hybrids that usually cannot reproduce sexually

Crossbreeding

• Combines superior traits and eliminates inferior trait of parent plants
• This process involves emasculation of one of the parent plants

Genetic Engineering

• Genetic engineering is performed by recombinant DNA technology
• Allows introduction of a foreign gene into a host organism

Genetic Engineering Advancements

• First performed by Cohen and Boyer by inserting a section of a plasmid of a bacterium into another bacterium
• Subsequent experiments introduced frog DNA into the bacterial genome

Recombinant DNA

• Crops are genetically engineered to possess various desirable traits to help global food security

Natural and Artificial Selection

• A population is a group of individuals that belong to a species, live in an area, and interbreed

Evolution

• Evolution is a process where the transformation of species happens through time
• Natural selection: individual organisms w/ favorable traits -> greater fitness + reproductive success
• Artificial selection: humans select plants or animals for breeding

Genetic Mechanisms of Population Change

• Population genetics studies genetic variation in populations
• It looks at the spatial and temporal variation of gene and allele frequencies

Traits

• Alleles: different versions of a gene
• Genotype: pairs of genes responsible for a trait
• Phenotype: physical expression of a trait

Mutation

• Advantageous mutations increase organism fitness
• Deleterious mutations decrease organism fitness
• Neutral mutations do not impact organism fitness

Genetic Drift

• Genetic drift is the change of allele frequencies as a result of random events in the environment
• Founder effect: loss of genetic variation in a new population established by few individuals
• Bottleneck effect: abrupt reduction caused by random environmental events

Reproductive Mechanisms of Speciation

• Reproduction: organisms create new individuals to carry genetic material from their parents
• Sexual reproduction: sex cells combine from both parents to create an offspring
• Asexual reproduction: produces offspring from a single organism
• Reproductive isolation: collection of evolutionary, behavioral, and physiological mechanisms which drive speciation
• Reproductive isolation: maintains integrity by reducing gene flow
• Prezygotic Barriers: inhibit individuals to copulate and have a successful fertilization event
  • Temporal: mating during different times of year
  • Ecological: occupying different habitats
  • Behavioral: different courtship behaviors
  • Mechanical: physical differences prevent copulation/pollination
• Postzygotic Barriers: inhibit individuals to have complete offspring
  • Hybrid inviability: failure to develop reproductive maturity
  • Hybrid infertility: failure to produce functional gametes
  • Hybrid breakdown: failure to develop properly

Modes of Speciation

• Speciation: emergence of new species from its ancestral population
• Evolution: transformation of species through time
• Speciation Rate: time for species to emerge
  • Punctuated Equilibrium
  • Gradualism
• Gradualism: changes that happen to species are usually small and accumulate through time
• Punctuated Equilibrium: traits are relatively stable in optimum environmental conditions, environmental changes appear, instability, results the development of new species
• Allopatric: geographically isolated populations
• Peripatric: a small population isolated at the edge of a larger population
• Parapatric: a continuously distributed population
• Sympatric: within the range of the ancestral population
• Allopatric Speciation: causes allopatric speciation because of river as geographic barrier
• Peripatric Speciation: involves the founder effect
• Parapatric Speciation: speciation in plants living in land mines, no specific barrier to inhibit gene flow

Sympatric Speciation

• Speciation of flies through sympatric speciation
• Sympatric speciation occurs within the same geographic location