Observing Patterns in Inherited Traits Mendelian Genetics.

Observing Patterns in Inherited Traits

Mendelian Genetics

Mendel’s Insight into Inheritance Patterns• More than a century ago people knew that traits were inherited, but

didn’t know that the information for these traits was contained in organized units called genes

• People thought that the traits from the mother and father were “blended” at fertilization• Blending could not explain the obvious variation in traits that

people could observe with their own eyes• “Blending” proponents dismissed Charles Darwin’s theory of

natural selection• Theory said that individuals of a population vary in the details

of the traits they have in common• Over generations, variations that help an individual survive

and reproduce show up among more offspring than variations that do not

• Less helpful variations might persist, but among fewer individuals – they may even disappear

Mendel’s Insight into Inheritance Patterns

• Gregor Mendel had guesses that sperm and eggs carry distinct “Units” of information about heritable traits

• By studying pea plants for generation after generation, Mendel found indirect observable evidence of how parents transmit genes to offspring

Mendel’s Experimental Approach

• Studies the garden pea plant• Self-fertilizing

• Male and female gametes originate in the same plant

• They can “true breed” for certain trains• This means that successive generation will be

just like parents in one or more trait. • All offspring grown from seeds of self-fertilized

white-flowered parent plants will also have white-flowers

• Pea plants can also cross-fertilize

Mendel’s Experiments

Terms used in modern genetics

• Remember that in Mendel’s time, no one knew about genes, meiosis, or chromosomes

• Genes – units of information about heritable traits transmitted from parents to offspring. Each gene has a specific location (locus) on a chromosome

• Cells with a diploid chromosome number (2n) have pairs of genes, on pairs of homologous chromosomes

• Mutation alters a gene’s molecular structure and its message about a trait. It may cause a trait to change, as when one gene for flower color specifies white and a mutant form specifies yellow.

• Alleles – all molecular forms of the same gene


• When offspring inherit a pair of identical alleles for a trait generation after generation, we expect them to be a true-breeding lineage. Offspring of a cross between two individual that breed true for different forms of a trait are hybrids; each one has inherited nonidentical alleles for that trait

• When a pair of alleles on homologous chromosomes are identical, this is a homozygous condition

• When the two alleles are not identical, this is called the heterozygous condition


• An allele is dominant when its effect on a trait masks that of any recessive allele pairs with it. We use capital letters to signify dominant alleles and lowercase letters for recessive ones. A and a are examples.

• A homozygous dominant individual has a pair of dominant alleles (AA) for the trait being studied

• A homozygous recessive individual has a pair of recessive alleles (aa).

• A heterozygous individual has a pair of nonidential alleles (Aa)


• Genotype refers to the particular alleles that an individual carries

• Phenotype refers to an individual’s observable traits.• P stand for parents

• F1 refers to the first-generation offspring

• F2 refers to the second-generation offspring

Mendel’s Theory of Segregation

• Mendel used something called a monohybrid cross to test his hypothesis that pea plants inherit two “units” of information for a trait, one from each parent.

• Monohybrid crosses use F1 offspring of parents that breed true for different forms of a trait

• Parents are AA crossed with aa (homozygous dominant cross with homozygous recessive)

• Offspring will all be heterozygous (the monohybrids)

• From meiosis these parents would produce A alleles and a alleles


Aa Aa

Aa Aa

A A

a

a

A = dominant allele for purple flowers

A = recessive allele for white flowers

All F1 offspring would be purple

AA = purple

Aa = purple

Mendel’s Theory of Segregation – Crossing the F1 Generation

AA Aa

Aa aa

A a

A

a

Results of this cross are 3 purple flowered plants and 1 white flowered plant a 3:1 ratio

AA = purple

Aa = purple

aa = white


• This 3:1 ratio indicated to Mendel that fertilization is a chance event having a number of possible outcomes

• Probability – that chance that each outcome of an even will occur is proportional to the number of ways in which the event can be reached

• These squares are called Punnett-square method

Possible Events Probable Outcome

Sperm A meets egg A ¼ AA offspring (Purple)

Sperm A meets egg a ¼ Aa offspring (Purple)

Sperm a meets egg A ¼ Aa offspring (Purple)

Sperm a meets egg a ¼ aa (white)


• C:\Documents and Settings\williamsonl\Desktop\hew06_int_mendelinherit.swf

• Mendel used rules of probability and counted many offspring• This minimized sampling error deviations in the

predicted results

Test Crosses

• Testcrosses supported Mendel’s prediction• Take an organism that shows dominance for a specific trait, but

its genotype is unknown• Cross it with a known homozygous recessive individual in a

number of matings• Results will show whether it is homozygous dominant or

homozygous recessive

• Mendel crossed F1 breeding plants with true-breeding white flowered plants (homozygous recessive)

• If the F1 plants were homozygous dominant then all of the offspring would be purple

• If the F1 plants were heterozygous then only about half the offspring would be purple

Testcrosses

Aa Aa

Aa Aa

a

a

A A

All Purple

Testcrosses

Aa aa

Aa aa

a

a

A a

Half Purple Half White


• Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of each pair are separated from each other during meiosis, so they end up in different gametes

Mendel’s Theory of Independent Assortment

• Used dihybrid crosses to explain how two pair of genes assort into gametes• We are now looking at two traits at a time

• An experimental intercross between F1 dihybrids (two traits) that are identically heterozygous for two pairs of genes

• http://wps.prenhall.com/esm_freeman_biosci_1/7/1948/498846.cw/index.html

• Theory became known as the theory of independent assortment

• As meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another, independently of gene pairs of other chromosomes

More Patterns than Mendel Thought

• Mendel was lucky, he focused (likely by accident) on traits that were clearly dominant and recessive forms

• Sometimes gene expression is not as straightforward• ABO blood types

• A case of codominance• A pair of nonidentical alleles affecting two

phenotypes are both expressed at the same time in heterozygotes

• ABO blood typing refers to the kind of glycolipid on the surface of the red blood cells

More Patterns than Mendel Thought• ABO blood types

• Three alleles IA, IB, i• IA and IB are codominant when paired together• i is recessive• If the letter for an allele has a superscript it show a lack of

dominance• Type A blood types

• IAIA, IAi• Type B blood types

• IBIB, IBi• Type AB blood types

• IAIB

• Type O blood types• ii

Blood Types Continued

• http://en.wikipedia.org/wiki/Blood_type

Incomplete dominance

• One allele of a pair is not fully dominant over its partner• Heterozygous phenotype is somewhere between the

two homozygotes • Snapdragons – Cross true-breeding red with true-

breeding white and you get pink flowers• Red snapdragons have two alleles that let them

make a lot of red pigment• White snapdragons have two mutant alleles

that produce no pigment• If you get one red and one white allele some

red pigment gets produced resulting in a pink flower

Incomplete dominance

• Same thing applies in chickens for their combs• R and P alleles

• Rose = RRpp• Pea = rrPP• Walnut = RrPp• Single = rrpp

Walnut

Single genes with a wide reach

• Pleiotropy – alleles at one locus on a chromosome that may affect two or more traits in good or bad ways• Typically seen in genetic disorders

• Cystic fibrosis• Sickle-cell anemia• Marfan Syndrome

When products of gene pairs interact

• Fur color in Labrador retrievers – depends on the amount of a protein called melanin that gets deposited in the hair• Black• Yellow• Brown

• Allele B (black) is dominant to b (brown)• Allele E permits full melanin deposition• Two ee alleles reduces it so the fur is yellow• Allele C may override the first two• CC or Cc do make an enzyme that allows melanin

production• cc does not allow for production and albinism or lack of

melanin is the result

Complex variation in traits

• Generally individuals of a population display a range of small differences in most traits• Called continuous variation• Depends mainly on how many gene products affect a

given trait and on how many environmental factors impact them.

• Greater number of genes and environmental factors, the more continuous is the distribution of all versions of the trait.

• Eye color• Height

Genes and the Environment

• Himalayan rabbit

• Homozygous for the ch allele of the gene specifying an enzyme involved in melanin production

• It is a heat sensitive enzyme that is active only when the temperature is below 33ºC or 91ºF.

• When or where it is warm the rabbit is grows white hair

• Where it is cold or cooler the dark hair appears

Observing Patterns in Inherited Traits Mendelian Genetics.

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