Fundamental Principles of Genetics (Chapter 8)

•Genetics – study of heredity

•Gene – determinant of heredity

•Chapter is outline of basic principles of genetics

Cell Theory of Inheritance

•Plants and animals composed of cells

•Plant cell – surrounded by cell wall

•Animal cell – surrounded by cell membrane

•Nucleus – location of chromosomes in cell

•Chromosomes – carrier of genes

Chromosomes

• Carriers of genetic material – genes

• Occur in pairs

• Homologous chromosomes – members of a pair

• Number of chromosomes constant among normal members of a species

Chromosome Number

• Diploid (2N) numbersdonkey 62 horse 64mule 63 swine 38sheep 54 cattle 60human 46 mink 30dog 78 cat 38chicken 78

• Normal body cells have diploid (2N) number• Gametes (sperm and egg) have haploid (1N)

number

Chromosomes

• Two major types

–Sex chromosomes – one pair which influences sex of organism

–Autosomes – all pairs other than sex chromosomes

http://learn.genetics.utah.edu/content/begin/traits/karyotype/index.html

Sex Determination

• Mammals–Sex chromosomes are:

• female XX homogametic• male XY heterogametic

• Chickens and turkeys– Sex chromosomes are:

• female ZW heterogametic• male ZZ homogametic

Chromosomal Abnormalities

• Changes in number of chromosomes (nondisjunction of sex cell)–Aneuploidy – extra or missing chromosome(s)

–Polyploidy – extra sets of chromosomes

• Usually lethal in animals–Except aneuploidy of very small chromosomes (eg Down Syndrome in humans is extra #21)

Chromosomal Abnormalities

• Changes in chromosome structure (see fig. 8.3, p. 143 illustration)–Deletion – piece of chromosome removed

–Duplication – piece of homologous chromosome duplicated

–Translocation – exchange of material between different (nonhomologous) chromosomes

–Inversion – segment of chromosome reversed

Chromosomal Abnormalities

• Changes in chromosome structure

–Cause loss of fertility

–Amount of loss dependent on type of change and importance of the affected segment of the chromosome

http://learn.genetics.utah.edu/content/begin/traits/karyotype/index.html

Cell Division

• Two types

–Mitosis – division of normal body cells

–Meiosis – division of germinal cells to produce gametes (sperm and egg)

Cell Division

• Mitosis

–Cell division of normal body cells

–Each daughter cell has same genetic complement as original cell

Cell Division

• Mitosis

–Four phases

• Prophase• Metaphase• Anaphase• Telophase

–Interphase – period between cell divisions

Cell Division

• Meiosis–Division in germinal cells to produce sperm and egg

–Each cell that results has 1N number of chromosomes (half of number of chromosomes in body cells

Cell Division

• Meiosis–Two cycles of:

• Prophase• Metaphase• Anaphase• Telophase

–Results in four cells with 1N number

Cell Division

• Meiosis–During late prophase I

• homologous chromosomes wrap together (synapsis)

– Crossing over – exchange of genetic material between homologous chromosomes

Cell Division

• Meiosis–Spermatogenesis – meiotic production of sperm cells

–Oogenesis – meiotic production of egg cells

Check It Out!

•http://www.dnaftb.org/

The Gene

• Component of DNA – deoxyribonucleic acid

• Large double stranded polymer of units called nucleotides

• Nucleotide –Sugar – deoxyribose–Phosphate group–Nitrogeneous base

• adenine guanine• cytosine thymine

Transcription of RNA

• Bases in DNA code for transcription of RNA

• RNA – ribonucleic acid–Also nucleic acid–Single stranded–Uracil instead of thymine–Ribose instead of deoxyribose

RNA

• Three primary types of RNA

• Messenger RNA–Transfers code from DNA to protein synthesis

• Transfer RNA–Carries amino acids to the ribosome

• Ribosomal RNA–Forms part of the structure of the ribosome

Translation of RNA

• RNA binds with ribosome

• Ribosome is location of protein synthesis

• Information on RNA used to generate amino acid sequence in proteins

Genetic Code

• Each amino acid coded by a three base sequence (codon)

• Most amino acids have more than one codon

• One codon specifies beginning of a protein

• Three codons specify the end of a protein

Control of Gene Function

• Complete gene is actually a complex of different types of genes (complex called an operon)

• Structural genes–Code for actual protein sequence

• Regulatory genes–Affect function of the structural genes

Interactions Between Genes

• Epistasis–Two or more gene pairs (not alleles) in which one gene pair influences expression of another gene pair – may or may not be on homologs

• Example:Horned, polled, scurred–Two gene pairs (horned vs polled) (smooth vs scurred)

–If horned – scurred vs smooth does not matter–If polled – scurred vs smooth can express

Genes and Embryological Development

• After union of sperm and egg–Cell division occurs

–Early development - all cells are alike

–Subsequent development – cells differentiate

–Embryological development is genetically controlled

Genes and Embryological Development

• Lethal genes–Genes which stop development

–Many must be received from both parents to cause death

–Cause of high percentage of embryonic death

Biotechnology

• Biotechnology–All technologies that pertain to molecular manipulation of living material

–Very difficult word to characterize

• Genetic engineering–New methods for modifying the animal genome

Microbe Engineering

• Recombinant DNA Technology–Segment of DNA removed from species of interest

–Segment is inserted into a microorganism

–Microorganism makes the product specified by the DNA

–DNA is “recombined” into a new location

Recombinant DNA

• Used for production of:–Bovine growth hormone to increase milk production

–Vaccines

–Other pharmaceuticals

–Bacteria to consume oil spills or other pollutants

Transgenesis

• Transgenesis–Movement of genes from one species into another

–First use – development of mouse with extra genes for growth hormone

Genetic Engineering in Plants

• Round-up-ready crops–Several crop species have been engineered to be resistant to the herbicide Round-up

• Bt crops–Corn and cotton have genes from a microbe that causes them to be resistant to corn borer and boll weevil

• Flavr-Savr tomato–Gene altered to lengthen shelf life

Cloning

• Clone – genetic identical–Identical twins are clones

–Cloning by embryo splitting has been available for several years

• Cloning from an adult DNA donor not possible until “Dolly”

Cloning

• Dolly–Developed at Roslin Institute

–Sheep cloned from cells from adult ewe mammary gland

–Cells had to be “started over” to remove results of cell differentiation

Cloning

• Potential uses for cloning in animals–Limited importance in routine livestock improvement

–Combine with transgenesis to produce animals with unique genetic makeup• transgenesis to create first copy of animal with

specific gene inserted • cloning to make multiple copies of that animal

Marker Assisted Selection

• Current selection –Based on actual traits measured in animals

• Marker assisted selection–Based on identification of genetic markers that are associated with performance traits

–Can be applied as soon as appropriate tissue (blood, skin etc) can be obtained

–Shortens time to obtain information for choosing superior parents

Segregation and Recombination

• Genes occur in pairs (except for those on sex chromosomes which are sex-linked)

• The segregation of genes (in body cells) at one locus (gene location) is normally independent of the segregation of genes (in gametes) at another location (Principle of Independent Assortment)

• Exception to this rule when genes are linked (on same chromosome)

Segregation and Recombination

• Linkage–Genes tend to be inherited together if close together on same chromosome

–Linkage relationships are broken by crossing over (exchange between homologous chromosomes) during meiosis I, prophase I

–Basis for idea of Marker Assisted Selection – if genetic marker is linked with gene for trait of interest, can select based on the marker

Mutation

• Mutation – change in the base sequence–May result in change in amino acid sequence in protein

–May result in change in the phenotype

–Change is usually detrimental (selection)

–Only source of new genetic material

Phenotypic Expression of Genes

• Phenotype = Genotype + Environment

• Gene action varies among different genes

• Dominance relationships vary widely

Phenotypic Expression of Genes• Dominant and recessive

• Color in AngusGenotype Phenotype

BB BlackBb Blackbb Red

–Black is dominant to red–Red is recessive

–BB or bb – homozygous–Bb - heterozygous

Phenotypic Expression of Genes

• Lack of dominance

• Color in Shorthorns

Genotype PhenotypeRR RedRW RoanWW White

• Both red and white are expressed in heterozygote

Phenotypic Expression of Genes

• Additive

• Each gene substitution (A for a, B for b) contributes the same amount (each A or B “adds” the same amount (absolute?)

Example: skin color in humansLivestock examples in Ch 9

• Contrast with non-additive types of gene action (dominance, epistasis)

Phenotypic Expression of Genes

• Genetic control of almost all traits is some combination of additive and non-additive

• Allows use of selection to improve herds through increasing frequency of desirable genes

• Allows use of crossbreeding to maximize efficient combinations of genes

Sex-linked Inheritance

• Some genes on the sex chromosomes

• Inheritance pattern affected because males have only one X chromosome

• Hemophilia in humans

Sex-influenced Inheritance

• Inheritance that is affected by sex of individual

• eg. scurs - dominant in males, recessive in females

• eg. horns in sheep – dominant in males, recessive in females

Sex-limited Inheritance

• Some traits express in only one sex

• Female–Milk production, egg production, age at first estrus

• Male–Scrotal circumference

Genome Project

• Human Genome project –Initiated by National Institutes of Health and Department of Energy

–Large multi-year, multi-location project to map the human genome

• Other genome projects –Cattle, sheep, swine, horses, dogs, cats, turkeys, chickens, mice and many other species