Basic Principles of Heredity

1. Basic Principles of Heredity Gregor Mendel (1822 – 1884) – Austrian monk who first discovered the basic rules of inheritance – his work was rediscovered in 1900 and he came to be known as the Father of Genetics

2. Definitions of Genetics Terms • locus – specific location of a gene on a chromosome • homologous chromosomes carry the same type of gene, located at the same place (same type, but may not be identical) • alleles – alternative forms of the same gene ex – human blood types produced by three different alleles A, B, and O • homozygous – an organism that has the same allele on both homologous chromosomes at a given gene locus • heterozygous – when the two homologous chromosomes have different alleles at a given gene locus – also called a hybrid

3. pure-breeding (true-breeding) – during Mendel’s time, commercial sources sold homozygous plants – Mendel started with these types of plants • phenotype – the physical appearance of an organism • genotype – the actual combination of alleles carried by an organism • genome – the whole of the genetic information of an organism

4. Mendel’s Experiments • worked with pea plants – advantages: • easy to grow • many contrasting traits to study • easy to control pollination • Mendel chose to study seven clearly contrasting inherited traits and subjected the results to mathematical analysis

5. Mendel began by crossing true-breeding plants with purple flowers to true-breeding plants with white flowers Purple x White P generation (parental) • F1 generation (first filial) all had purple flowers • Mendel said that the purple trait was dominant – it covered up the white trait • he said the white trait (the one hidden) was recessive

7. Mendel then crossed the F1 generation • Purple x Purple F1 • F2 generation (second filial) were a mixture of purple and white

8. Inheritance of Dominant and Recessive Alleles – How can we explain Mendel’s results? • Each trait is determined by pairs of genes. Each individual has two genes for each trait, one on each homologous chromosome (one from mom, one from dad). • Mendel’s Law (Principle) of Segregation – pairs of genes on homologous chromosomes separate from each other during gamete formation.

9. Each gamete receives only one of each parent’s pair of genes for each trait. When a sperm fertilizes an egg, the offspring receives one allele from the father one from the mother • When two alternative forms of a gene are inherited, one (dominant) may mask the expression of the other (recessive) but it does not change the recessive allele. The unchanged recessive allele may be passed to offspring in the individual’s gametes

10. Monohybrid Crosses - inheritance of two alleles of a single locus • We use alphabet letters to represent alleles. Capital letters for dominant traits and lower case for recessive traits. Always use the same letter for the same trait. ex - PP x pp P generation P – purple p - white F1 are all Pp

11. Punnett square – used to figure out the possible combinations of eggs and sperm at fertilization

12. a cross between a homozygote and a heterozygote always results in a 1:1 genotypic ratio for example: P PP x Pp F1 1 PP: 1 Pp P pp x Pp F1 1 pp: 1 Pp

13. phenotype does not always reveal the genotype • ex – heterozygotes – must do a test cross to determine genotype • test cross – an individual of unknown genotype showing a dominant phenotype is crossed with an individual who is homozygous recessive

15. Sex Determination • most animals have a special pair of sex chromosomes (all other chromosomes are autosomes) • members of one sex are homogametic – have a pair of similar sex chromosomes and produce only one type of gamete • females of many species (including humans) have two X chromosomes – form only “X” gametes • genotype is XX • members of the other sex are heterogametic – have two different sex chromosomes • males of many species (including humans) have one X chromosome and one Y chromosome – form ½ “X” gametes and ½ “Y” gametes

16. all individuals require at least one X chromosome and the Y is the male-determining chromosome • X and Y chromosomes are not truly homologous • different in shape, size, and genetic constitution • Male determines the sex of the baby • ½ the sperm are Y and ½ are X • all of the eggs are X • if an X sperm fertilizes the egg, the baby is XX and is a girl • if a Y sperm fertilizes the egg, the baby is XY and is a boy

17. Sex-linked traits • genes that are on one sex chromosome but not on the other (also called X-linked) • Y chromosome carries few genes other than those that determine maleness • X chromosome bears many genes that have nothing to do with being female (has genes for color vision and blood clotting) • females receive two alleles for traits found on the X chromosome (one from mom and one from dad) • she can be either homozygous or heterozygous • males only receive one allele for traits on the X chromosome (from mom) because the other chromosome is Y (which came from dad) and does not carry the X traits • males express all of their X chromosome alleles whether or not they are dominant (because there is only one X chromosome)

18. Sex-linked traits are usually expressed in the male • Example: Color vision is inherited on the X chromosome • if a female inherits the recessive gene for colorblindness, she still has a second gene to possibly make up for it Xc XC • a female must inherit two recessive genes (one from mom and one from dad) in order to express a sex-linked trait Xc Xc • males only have one X chromosome (comes from mom) – if the allele on that chromosome is recessive, it will be expressed (he will be colorblind) Xc Y

20. Gene Interactions – many traits are not just controlled by one pair of genes that are dominant or recessive – many traits are controlled by many pairs of genes cooperating to control the expression of a single trait 1. Incomplete dominance – alleles are not always completely dominant or recessive • crossing red snapdragons with white snapdragons produces all pink offspring • when the heterozygote has a phenotype that is intermediate between those of the two parents, the genes show incomplete dominance

22. 2. Codominance – the heterozygote simultaneously expresses the phenotypes of both homozygous parents (ex. roan coat color in horses and cows, human blood types) 3. Multiple alleles – when three or more alleles exist for a given gene within a population (ex. eye color in fruit flies, human blood types) • any one individual has only up to two alleles

23. Human Blood Types • involves 3 alleles – A, B, O • produces 4 blood types: A, B, AB, O • blood types are based on antigens (surface proteins) on the red blood cells

24. exposure to an antigen causes an immune response – the body produces antibodies to destroy the antigen • immune system of each person is insensitive to the surface proteins (antigens) of its own cells blood typeantigensplasma antibodies A A anti-B B B anti-A AB A and B none O none anti-A and B • if a person receives the wrong blood type in a transfusion, the antibodies react with the antigen and cause agglutination (clumping) of cells

25. 1.type A can receive O, A 2.type B can receive O, B 3.type AB can receive all types (universal recipient) 4.type O can receive only O (universal donor)

26. Inheritance of blood types • 3 alleles of the same gene: A (IA), B (IB), and O (i) • IA and IB are codominant • IA and IB are both dominant over i phenotypegenotype type O ii type A IAIA or IAi type B IBIB or IBi type AB IAIB