Patterns of Inheritance

Patterns of Inheritance

PATTERNS OF INHERITANCE Genetics Science of heredity Dog breeds are a perfect way to look at genetics and the inheritance of traits About 15000 years ago in Asia, humans began to cohabitate with wild canines The humans moved and began to breed the wild dogs for their specific needs. Certain traits were passed on

Today there are over 85 recognized breeds of dogs, all coming from that one ancestral canine History of Genetics The ancient physician Hippocrates first tried to explain inheritance Suggested pangenesis Particles (pangenes) from each part from an organisms body travel to the eggs or sperm and then are passed on to the next generation

Aristotle rejected pangenesis Stated that what is inherited is the potential to produce body features, not the particles of the features themselves History of Genetics Pangenesis is incorrect The reproductive cells are not composed of particles from the somatic (body) cells Changes in somatic cells do not influence eggs and

sperm Ex. weightlifting Biologists early in the 19th century established that offspring inherit traits from both parents Used plants History of Genetics The new hypothesis was the blending hypothesis

Hereditary material from both parents were blended together forming the offspring If this was the case then when a brown and black Labrador retriever were bred, then the brown and black colors should mix. This obviously isnt how it works! This hypothesis was rejected because it didnt explain why some traits can disappear in one generation then reappear in a later generation!

Modern Genetics Modern genetics began in the 1860s with an Austrian monk named Gregor Mendel. Bred garden peas Chosen for their short generation times, large numbers of offspring, and came in easily distinguishable varieties Argued that heritable factors (genes) retain their individuality

Do not mix or blend Mendel Genetics The pea plants had different colored flowers Flower color is considered a character Each variant for a character, such as purple or white, is called a trait Self-fertilization Plants male pollen fertilizes that plants eggs

Cross-fertilization Pollen from one plant fertilizes eggs from another plant Mendel Genetics Mendel Genetics Chose 7 characteristics to study Bred the characteristics until he was sure he had true-breeding varieties

Varieties that produced offspring all identical to the parent Ex. purple flower plant, when selffertilized, will always produce all purple flowered offspring Mendel Genetics What would happen if different true-breeding plants were crossed? Purple flower with white flower? Offspring of two different truebreeding individuals are

considered hybrids Cross-fertilization is called hybridization or a cross Parents are called the P generation Their hybrid offspring are called F1 generation If the F1 generation is self-fertilized or fertilize each other, the resulting offspring is called the F2 generation Mendels Law of

Describes inheritance of a single character Segregation Ex. Flower color Example of a monohybrid cross (cross of plants with only one different character) Parental generation: purple flowers white flowers F1 generation: all plants with purple flowers F2 generation: of plants with purple flowers of plants with white flowers

Mendel needed to explain Why one trait seemed to disappear in the F1 generation Why that trait reappeared in one quarter of the F2 offspring Mendel saw this pattern in each of the 7 characteristics P generation

(true-breeding parents) Purple flowers White flowers P generation (true-breeding parents) Purple flowers F1 generation

White flowers All plants have purple flowers P generation (true-breeding parents) Purple flowers White flowers

F1 generation All plants have purple flowers Fertilization among F1 plants (F1 F1) F2 generation 3

4 of plants have purple flowers 1 4 of plants have white flowers Mendels Law of

From this, Mendel came up with four hypotheses Segregation 1. Genes are found in alternative versions called alleles genotype the listing of alleles an individual carries for a specific gene 2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different A homozygous genotype has identical alleles A heterozygous genotype has two different alleles

Mendels Law of Four hypotheses (cont) Segregation 3. If the alleles differ, the dominant allele determines the organisms appearance, and the recessive allele has no noticeable effect phenotype the appearance or expression of a trait 4. Law of segregation: Allele pairs separate (segregate) from each other during the

production of gametes (meiosis) so that a sperm or egg carries only one allele for each gene Mendels Law of Segregation Mendels Law of Segregation How can we determine the how likely it is for a

specific trait to be passed on? Punnett Square Shows proportions of genotypes and phenotypes in the next generation Alleles for one parent go on top and the other parents alleles go on the side P P P=dominant (purple) p=recessive (white)

p P Punnett Square The allele for each parent is then placed in the squares below and next to it. Monohybrid cross P

P P PP PP p Pp

Pp Punnett Square The resulting offspring: Genotype 2 are homozygous dominant 50% 2 are heterozygous 50% Phenotype P All four have purple flowers 100%

P p PP PP Pp Pp P

Punnett Square What happens if we cross two heterozygous parents? P P p p Punnett Square

What happens if we cross two heterozygous parents? P p P PP p Pp

Pp pp Punnett Square Offspring genotype: 1 homozygous dominant 25% 2 heterozygous 50% 1 homozygous recessive 25% P P

p PP Pp p Pp pp

Punnett Square Offspring phenotype: 3 have purple flowers 75% 1 has white flowers 25% P P p PP Pp

p Pp pp Punnett Square Ratios: Genotype ratio: 1PP:2Pp:1pp Phenotype ratio: 3purple:1white

P p P p PP Pp Pp

pp Homologous chromosomes bear alleles for each character Each diploid cell has two sets of homologous chromosomes One comes from the female parent, and one comes from the male parent

Homologous chromosomes have genes for the same characters located at the same positions along their lengths Homologous chromosomes bear alleles for each character Law of independent assortment Two of the characters Mendel studied were

seed shape (round or wrinkled) and seed color (green or yellow) He knew that the allele for round seeds were dominant (R) to the allele for wrinkled seeds (r) The allele for yellow seeds are dominant (Y) to the allele for green seeds (y) What would be the results if Mendel crossed plants differing in two characters?

Law of independent assortment Need a dihybrid cross Mendel crossed plants having round yellow seeds (RRYY) with plants having wrinkled green seeds (rryy) Resulted in hybrids heterozygous for both characteristics (RrYy) All had round yellow seeds Were the two characters from the parents

packaged together or was each character inherited independently of the other? Law of independent assortment Mendel allowed fertilization among the F1 plants What will the next generation look like? Law of independent

assortment If the genes were inherited together, then the F1 generation would only produce the same two genotypes (RY and ry) The resulting phenotype ratio should be 3 round yellow seeds : 1 wrinkled green seed If the genes are inheretited independently, then the F1 generation would produce four

gamete genotypes (RY, rY, Ry, and ry) in equal quantities. Law of independent assortment Law of independent assortment 9 genotypes result RRYY, RrYY, RRYy, RR yy, RrYy, rrYY, rrYy, Rryy, Rryy, rryy

4 phenotypes result in a ratio of 9:3:3:1 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green Law of independent assortment A dihybrid cross is equivalent to two monohybrid crosses happening simultaneously. From the 9:3:3:1 ratio we get

12 plants with round seeds and 4 with wrinkled seeds 12 plants with yellow seeds and 4 with green seeds This reduces to 3:1 same as a monohybrid cross These results support the hypothesis that each pair of alleles segregates independently of other pairs during gamete formation. Testcross We know that a chocolate lab has a genotype

of bb for coat color. What about a black lab? Can be BB or Bb How can we determine the dogs actual genotype? Testcross Perform a testcross Cross an unknown genotype with a homozygous

recessive If the black lab is homozygous dominant, all of the offspring will be black (genotype of Bb) If the black lab is heterozygous, then the offspring should be half black and half chocolate (genotype of Bb and bb) Rules of probability The segregation of alleles obey the rules of

probability. Same as tossing coins, rolling dice, etc. Probablility scale is from 0 to 1 1=certain to occur 0=certain not to occur With a coin, the chance of heads is and the chance of tails is With a deck of cards, the chance of pulling the

ace of spades is 1/52 The chance of pulling any other card is 51/52 Rules of probability It is important to realize that previous results do not affect the probability of future results The chance of heads when flipping a coin is ALWAYS regardless of the results of previous flips Called independent events

If two coins are flipped simultaneously, the outcome for each coin is independent of the other. One coin does not influence the other Rules of probability What is the chance that BOTH coins will land heads up? This is called a compound event

The probabilities for this type event is the product of the probabilities of each independent event. For the coins, x = This is called the rule of multiplication Holds true for genetics as well as coin tosses Rules of probability Rules of probability

What is the probability that an F2 Labrador will be heterozygous for the coat-color gene? There are 2 ways F1 gametes can combine to produce a heterozygous offspring The dominant allele (B) can come from the female egg and the recessive allele (b) can come from the male egg, and vice versa

Rules of probability The probability that an event can occur in two or more alternative ways is the SUM of the separate probabilities of the different ways. This is the rule of addition We calculate the probability of an F2 heterozygote as + = 1/2

Rules of probability What would be the probability if we looked at three characters (trihybrid cross)? Aa x Aa: probability of aa offspring = Bb x Bb: probability of bb offspring = Cc x Cc: probability of cc offspring = Probability of aabbcc? aa x bb x cc = 1/64 Incomplete dominance In Mendels experiments, the offspring always

looked like one of the two parental varieties This is called complete dominance The dominant allele has same phenotypic effect whether present in one or two copies The seeds were either yellow or green, smooth or wrinkled with nothing in between The appearance of some characters falls between the two parental varieties

This is called incomplete dominance Incomplete dominance If a red snap dragon is crossed with a white snapdragon, all the F1 hybrids have pink flowers This does not support the blending theory however

Check the punnett square for the F2 generation Codominance There is also another anomoly where neither trait is dominant over the other.

In this case, the offspring present a mixture of both traits. Comparing Incomplete Dominance to Co-dominance Multiple alleles So far we have discussed inheritance involving two alleles per gene. Many genes can be found in more than two

versions, known as multiple alleles Blood is an example Involves three alleles IA, IB, i In combination produces four phenotypes Multiple alleles The four blood groups result from the various

combinations of the three different alleles IA for enzyme that adds the carbohydrate A IB for enzyme that adds the carbohydrate B i for neither A or B Both IA and IB are dominant to i IA and IB are codominant because both alleles are expressed in the heterozygous individual Different than incomplete dominance Multiple alleles

Pleiotropy Pleiotropy when a single gene affects multiple characters Sickle-cell disease Blood cells produce abnormal hemoglobin molecules Polygenic inheritance Polygenic inheritance

Multiple genes influencing a single character Skin color and height Environmental factors Many characters result from a combination of heredity and environment. Sun exposure influence skin color, exercise alters

body build, experience improves performance on intelligence tests. Cancer and heart disease can be affected by both heredity and lifestyle Chromosome theory of inheritance The theory states that genes occupy specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis.

The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns Chromosome theory of inheritance Chromosome theory of inheritance Chromosome theory of inheritance

Linked genes Genes that are located close to each other are generally inherited together Linked genes Do not follow Mendels law of independent assortment. Crossing Over Increases genetic variability

Happens in prophase I Genetic recombination is the production of new combinations of genes due to crossing over % of recombinations is called recombination frequency Crossing over involves exchange of genetic material between homologous chromosomes Nonsister chromatids join at a chiasma (chiasmata), the site of

attachment and crossing over Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids Tetrad Chiasma Centromere

Crossing over Sex Chromosomes Sex chromosomes chromosomes that determine an individuals sex

Designated X and Y XX = females; XY = males Humans have 44 autosomes (nonsex chromosomes) Human gametes contain one sex chromosome and a haploid set of autosomes (22) All eggs contain a single X chromosome Half of sperm cells contain an X, and half contain a Y Sex of offspring depends on whether a sperm cell with an X or Y fertilizes the egg Sex Chromosomes

Sex Chromosomes One gene on the Y chromosome is vital role in determining sex SRY triggers testes development The absence of SRY allows ovaries to develop Sex-Linked Genes Sex-linked gene a gene located on either sex chromosome

Example in fruit flies Red eyes are dominant, white eyes are recessive and rare Because the alleles are carried on the X chromosome, they are shown as a superscript. XR and Xr Red-eyed males: XRY ; White-eyed males: XrY Red-eyed females: XR XR or XRXr ; White-eyed females: XrXr Sex-Linked Genes

Fruit fly example (cont) A white-eyed male will transmit is X r to all of his daughters, but not his sons because the Y gene necessary for male offspring doesnt determine eye color. Sex-Linked Genes Most sex-linked disorders affect mostly males A male only has to receive one sex-linked

recessive allele for the disorder to be exhibited A female must inherit two sex-linked recessive alleles for the disorder to be exhibited Hemophelia, color blindness, and muscular dystrophy are examples of sex-linked disorders in humans. Sex-Linked Genes

The Y chromosome provides clues about human male evolution Without a mutation, the Y chromosome passes intact from father to son. Allows researchers to learn about the ancestry of human males. The ancestry of about 8% of central Asian men (16 million) was traced back to a single male living about

1000 years ago. Genghis Khan 10% of Irish men can be traced back to a warlord that lived during the 5th century Niall of the Nine Hostages Review of Types of Dominance 1.Complete dominance- one allele completely masks the other

2.Incomplete dominance- one allele doesnt completely mask the other - result is somewhere in between 3.Codominance- neither allele completely masks the other - result is a little of each 4.Multiple alleles- more than one allele for a trait 5.Polygenic traits- more than two genes control a trait 6.Sex-linked gene- gene is located on the X chromosome Inherited disorders Usually controlled by a single gene Most genetic disorders are recessive Affected people are born to normal parents who

are both heterozygotes They carry the gene, but are phenotypically normal We can predict the fraction of affected offspring likely to result from a mating between two carriers. Inherited disorders (recessive) Suppose two heterozygous carriers for

deafness (Dd) had a child What is the probability that the child would be deaf? Inherited disorders (recessive) Cystic fibrosis (CF) Affects 30,000 children in the US and 70,000 worldwide

The CF allele is carried by about one in 25 people of European ancestry A person with two copies of this allele will have CF Characterized by excessive secretion of a thick mucus from lungs, pancreas and other organs Can interfere with breathing, digestion and liver function Person is vulnerable to bacterial infections No cure, but treatment can allow a person to live until about 37 Inherited disorders (recessive)

Inbreeding Two individuals with recent ancestors are more likely carry the same harmful recessive gene If they mate, the chances of an offspring with homozygous recessive trait increases. Inbreeding is illegal and/or taboo in many societies Inherited disorders (dominant) Some disorders are caused by dominant

alleles Some are non-lethal: extra fingers or toes (polydactyly) or webbed fingers and toes Some are serious: achondroplasia A form of dwarfism Head and torso develop normally, but arms and legs are short. 1/25,000 are affected Inherited disorders (dominant)

Achondroplasia cont The homozygous dominant genotype causes death in embryo Only people with heterozygous genotypes have the disorder Someone with disorder has 50% chance of passing it on. So all those who do not have achondroplasia (99.99% of population) are homozygous recessive Shows that dominant doesnt always mean more

common Inherited disorders (dominant) Dominant alleles that are lethal are much less common than lethal recessive alleles Why? Dominant alleles cant be carried by heterozygotes without affecting them

Many of these kill the embryo or dont allow the person to live long enough to pass on his/her genes Gene gets eliminated People who carry recessive disorders live with recessive genes without knowing it and pass them on. Inherited disorders Testing for genetic disorders

Genetic testing Tests can determine if a prospective parent doesnt carry a harmful recessive allele or is heterozygous Can then calculate chances of having an affected child Testing for genetic disorders Fetal testing Requires the collection of fetal

cells Amniocentesis Performed between weeks 14-20 of pregnancy Doctor inserts a needle into the womb and extracts about 10ml of fluid. DNA is checked for abnormalities Chorionic villus sampling (CVS) Doctor extracts small sample of placenta DNA is checked for abnormalities Both carry risk of complications to pregnancy

Testing for genetic disorders Fetal imaging Ultrasound Uses soundwaves to produce a picture of a fetus Growth and development can be assessed Karyotype Shows stained and magnified versions of chromosomes Karyotypes are produced from dividing white

blood cells, stopped at metaphase Karyotypes allow observation of Homologous chromosome pairs Chromosome number Chromosome structure Hypotonic solution Packed red and white blood cells

Blood culture Fixative Stain Centrifuge 2 White blood cells

3 1 Fluid 4 5Pair Sisterof homologous chromatids

chromosomes Centromer e Pedigrees Allows for genetic traits in humans to be tracked Males are represented by a square Females are represented by a circle Individuals affected with the trait are filled in

Individuals not affected with the trait are left open Can be used for any trait that is simple dominant/recessive, controlled by a single gene Examples: Freckles, Widows Peak, and Earlobes Pedigrees

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