Genetics Genetics is the science of heredity Genetics explains how genes bring about characteristics in living organisms and how those characteristics are transmitted from parents to offspring Genetics is at the center of all biology because genes of an organism determines all biological processes

Genetics Genes are discrete units of inherited information consisting of a specific nucleotide sequences within molecules of DNA Each gene provides information related to a particular trait (protein) that the organism exhibits

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Experimental Genetics The modern science of genetics began in the 1860’s when Gregor Mendel, an Austrian monk studied the principles of genetics by breeding garden peas available in a wide variety of shapes and colors cheap and abundant short generation times with large amounts of offspring

Experimental Genetics Mendel studied 7 characters (heritable features) of pea plants each with its own distinctive trait (variant of that character) He created true-breeding lines of peas that demonstrated only 1 variation of a particular trait over many generations for example, a true breeding purple pea plant had only purple flowers, not white

Flower color Purple White Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Stem length Tall Dwarf

Experimental Genetics Mendel wanted to see what happened when he crossed true-breeding lines for one trait with true-breeding lines for the opposite trait (purple vs white, round vs wrinkled, tall vs dwarf) His results led to the establishment of several principles: Mendel’s Law of Dominance Mendel’s Law of Segregation Mendel’s Law of Independent Assortment

What happens when you cross a true breeding plant that has purple flowers with a true breeding plant that has white flowers? 100% of the plants following the cross have purple flowers (no plant had white flowers)

What happens when you cross these 2nd generation purple flowered plants with one another? 75% of the plants following the cross have purple flowers and 25% of the plants following the cross have white flowers 9

Mendel’s Law of Dominance The white and purple flowers of the pea plants are two versions of a gene for flower color Alternative versions of a gene are called alleles Mendel’s law of dominance states that when an organism has 2 different alleles for any given character, 1 allele will dominate over the other the dominant allele is more common in the population Dominant alleles are designated with a capital letter, whereas their counterpart recessive allele is designated with the same letter, but lowercase

P plants Gametes F1 plants (hybrids) Gametes F2 plants Genetic makeup (alleles) P plants PP pp Gametes All P All p F1 plants (hybrids) All Pp Gametes 1 – 2 P 1 – 2 p F2 plants P p Phenotypic ratio 3 purple : 1 white P PP Pp Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp

Mendel’s Law of Dominance For each character, an organism inherits 2 alleles, 1 from each parent These alleles may be the same or different An organism that has 2 of the same alleles for a trait is said to be homozygous An organism that has 2 different alleles for a trait is said to be heterozygous

Mendel’s Law of Dominance If the 2 alleles of an inherited pair differ (heterozygous), then one allele will determine the organism’s appearance over the other, and is called the dominant allele The other allele has no noticeable effect on the organism’s appearance and is called the recessive allele

Mendel’s Law of Segregation A sperm or egg carries only 1 allele (haploid) for each inherited trait This is because allele pairs segregate (separate) during gamete formation (meiosis) When sperm and egg unite during fertilization, they each contribute their own allele, restoring the paired ‘condition’ (diploid) to the offspring

P a B P a b 15

Mendel’s Law of Independent Assortment The alleles of a gene pair separate from one another independently of the other alleles of another gene pair during segregation (meiosis) The origin of any particular allele will be randomly selected from paternal or maternal chromosomes via the process of crossing-over explains why the shape of a pea (round vs wrinkled) is independent of its color (green vs yellow) peas can be green and round or yellow and round or green and wrinkled or yellow and wrinkled

In this example, yellow and green are 2 traits for the color character (indicated by Y and y, respectively) and round and wrinkled are 2 traits of another character (indicated by R and r, respectively) RRYY rryy Gametes RY ry RrYy Sperm 1 – 4 RY 1 4 – rY 1 4 – Ry 1 4 – ry 1 – 4 RY RRYY RrYY RRYy RrYy 1 4 – rY RrYY rrYY RrYy rrYy Yellow round 1 – 4 Ry 16 –– 9 RRYy RrYy RRyy Rryy Green round 16 –– 3 1 – 4 ry Yellow wrinkled RrYy rrYy Rryy rryy 16 –– 3 Green wrinkled 1 16 ––

The Chromosome Basis of Inheritance Mendel established his principles (laws) of inheritance long before mitosis and meiosis were understood, and longer still before chromosomes were ‘discovered’ The chromosome theory of inheritance states that genes occupy specific loci, or location on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis 18

Fertilization among the F1 plants All round yellow seeds (RrYy) F1 generation R y r Y R r r R Metaphase I of meiosis (alternative arrangements) Y y Y y R r r R Anaphase I of meiosis Y y Y y R r r R Metaphase II of meiosis Y y Y y Gametes y Y Y y Y Y y y R R r r r r R R 1 – 4 ry 1 – 4 rY 1 4 – Ry 1 4 – RY Fertilization among the F1 plants F2 generation 9 :3 :3 :1 19

Genetics Terminology The complete genetic make-up of an organism is called its genotype The physical expression of the genotype is its phenotype P a B Genotype: P a b PP aa Bb Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous

Mendel’s laws reflect the rules of probability Mendel’s strong background in mathematics (and physics and chemistry…) served him well in his studies of inheritance He knew he needed large sample sizes The laws of inheritance reflect the probability of an event occurring The probability of having a girl: 1 in 2 The probability of rolling a 5 on a dice: 1 in 6 The probability of drawing a queen from a deck of cards: 4 in 52 (1 in 13)

Probability An event that is certain to occur has a probability of 1 or 100% An event that is certain not to occur has a probability of 0 or 0% When you flip a coin, the probability of getting heads (or tails) is 1 in 2 (50%) every time you toss the coin independent of previous tosses

Segregation and fertilization as chance events Bb male Formation of sperm Bb female 1 – 2 B 1 – 2 b Formation of eggs B B B b 1 – 2 B 1 – 4 1 – 4 b B b b 1 – 2 b 1 – 4 1 – 4 F2 genotypes

Genetic traits may be tracked Individuals exhibiting a recessive trait would be homozygous recessive (carry 2 copies of the recessive allele) Individuals exhibiting a dominant trait, however, could be homozygous dominant (carry 2 copies of the dominant allele) or be heterozygous (carry 1 copy of the dominant and 1 copy of the recessive allele)

Genotype Genotype F_ ff W_ ww ee E_ Dominant Traits Recessive Traits Freckles No freckles W_ ww Widow’s peak Straight hairline ee E_ Free earlobe Attached earlobe

Genetic disorders Genetic disorders may be inherited as a recessive or dominant trait Most human genetic disorders are recessive; most people who have recessive disorders are born to normal parents who are both heterozygous for the allele controlling the disorder In this way, the parents are carriers of the recessive allele, but are phenotypically normal

Offspring produced by parents who are carriers for a recessive trait Normal Dd Normal Dd Parents D Sperm d Dd Normal (carrier) DD Normal Does this mean that deaf parents always have deaf children? D Eggs Dd Normal (carrier) dd Deaf d Offspring

Three’s a crowd… Mendel was fortunate in that he chose characters for which there were only 2 alleles Many genes, however, have more than 2 alleles in the population More often than not, the inheritance patterns of a particular trait are more complex

Incomplete Dominance In some allele combinations, dominance does not exist Instead, 2 traits are blended together to form a 3rd trait In snapdragons when a red flowering plant is crossed with a white flowering plant, some offspring are red, some are white and some are pink

Red RR White rr P generation Gametes R r F1 generation Pink Rr Gametes – 2 R 1 – 2 r Sperm 1 – 2 R 1 – 2 r F2 generation RR rR 1 – 2 R Eggs Rr rr 1 – 2 r

Incomplete Dominance In this case, heterozygous individuals exhibit a third phenotype, pink. The resulting pink flowers are Rr and can produce red, white or pink offspring of their own In the case of incomplete dominance, the phenotype does reveal the genotype for all traits

Multiple alleles Multiple alleles exist for most genes For example, the ABO blood group in humans involves 3 alleles of a single gene: A, B, and O An individual can have type A, B, O, or AB blood The A and B alleles are co-dominant; both alleles are expressed in heterozygous individuals

Blood Group (Phenotype) Genotypes Red Blood Cells O OO AO or AA A Carbohydrate A BO or BB B Carbohydrate B AB AB

Multiple alleles No matter how many alleles for a given gene are in a population, a diploid individual will only have 2 alleles, one on each homologous chromosome Homozygous for the dominant allele recessive allele Heterozygous Genotype: P B a PP aa b Bb