Chapter 13: Patterns of Inheritance

Genes and Chromosomes

              I.      13.1 Heredity and Environment

a.       Affects of the environment on heredity

                                                               i.      Ex. Siamese cats

1.      Siamese cats inherit genes for enzymes that produce a dark pigment in their fur.  The pigment producing enzyme works best at cool temperatures thus their legs, tails and ears darken when the cat is let out into the cool outdoors.  Their fur color could change from dark to light if a body part was kept warm (inside all the time), and vice versa.

                                                             ii.      Ex. Twins

1.      Identical twins develop when a zygote splits and forms two complete embryos with the exact same genetic information.

2.      Fraternal twins develop from separate eggs and sperm cells and don't have the exact same genetic information (two were released during that menstrual cycle rather than the usual one).

3.      If identical twins have more of the same trait than fraternal twins, then the trait's influenced by genetics.  If the trait is different in the identical twins, then the trait's influenced by the environment.

b.      Theory of Blending [old and outdated, little supporting data]

                                                               i.      Genetic makeup of someone is formed by their parents’ traits getting mixed (blended) during fertilization.

                                                             ii.      After the blending, traits couldn't be passed down separately to the next generation.

                                                            iii.      This theory is no longer included in genetics because traits are not blended, even though many children have intermediate traits of their parents.  Those that appear to be blended are really polygenic traits (coded for by many different genes).

           II.      13.2 Mendel and the Idea of Alleles

a.       Mendel’s experiments with peasmendel

                                                               i.      Mendel decided to study traits that didn't fit into the blending theory.

1.      Smooth vs. wrinkled

2.      Tall vs. short

3.      Green pods vs. yellow pods etc….

                                                             ii.      Mendel used true-breeding peas to produce offspring that were identical to themselves for many generations.  [round seeded peas bred together always produce round seeds].

                                                            iii.      Mendel crossbred all the pea plants and classified the offspring.  Found 3:1 ratios in his F2 generations for many traits.

1.      Yellow x Green = greenish yellow?  Both green and yellow?  Results showed both green and yellow pods in the next generations, not blending.



All green-podded

self fertilization

ratio: 3 green podded: 1 yellow podded


b.      Genetic theories today

                                                               i.      One gene codes for 1 protein or trait

                                                             ii.      Ex. Human alleles

1.      widow's peak

2.      hair/skin color

3.      nose shape

         III.      13.3 Genes and Chromosomes

a.       Arrangement of genes in Eukaryotes

                                                               i.      Coding DNA

1.      Long molecules of DNA that are wrapped around proteins (complex molecules called chromosomes).

2.      Only 1% of DNA is expressed in a protein. 

                                                             ii.      Non-Coding DNA
      1.   Introns

2.      Is not translated

3.      Consists of short sequences of bases that are repeated thousands of times.


b.      Arrangement of genes in Prokaryotes

                                                               i.      Coding DNA

1.      Bacteria (Monera, Prokaryotic) have 1 chromosome that is usually circular, and is associated with only a small amount of protein.

2.      Donut have introns.

                                                             ii.      Non-Coding DNA

1.      90% of DNA in prokaryotes is translated.

c.       Plasmids

                                                               i.      Small circles of DNA in bacteria that contain additional genes.

                                                             ii.      Move from 1 bacterium to another

                                                            iii.      Genetic engineers use them to introduce genetic material into bacteria cells.  Called: genetic engineering.

                                                           iv.      Carry genes that provide resistance to antibiotics.

d.      Homologous structures vs. other chromosomes

                                                               i.      Homologous chromosomes carry the same genes, but may be present as different alleles.  For example, brown eyed people can pass on the recessive blue eyed gene they carry to their offspring.

                                                             ii.      Size of a chromosome and the position of its centriole.

1.      Stains can help by binding to specific regions of chromosomes, providing a pattern that is specific to each of an organism's chromosomes.

e.       Studying chromosomes during cell division

                                                               i.      Chemicals are added to a cell to stop cell division during metaphase.

                                                             ii.      Cells are placed on slides and treated with water so that they swell, and their chromosomes spread apart.

                                                            iii.      Stains help produce the banding patterns that are used to study the chromosome.

Biological Challenges

The Human Genome Project

This project involves thousands of different scientists and technicians, and has cost over $1 billion.  This project has determined the sequence of the pairs of DNA in the human genome.  The human genome consists of 23 numbered pairs of chromosomes (46 total chromosomes).  Bioinformatics is the method of analyzing the vast amount of sequence data generated by the HGP.
  Mendelian Patterns of Inheritance

              I.      13.4 Probability and Genetics

a.       What is probability?

                                                               i.      Method of predicting the chances of how (if) an event will occur.

1.      Geneticists use it to predict the results of mating.

                                                             ii.      Ex. What's the chance of a tossed coin to land on heads? 1 out of 2, ½ or 50%..

Ex. What's the chance of you drawing an ace out of a deck of cards? 4 out of 52, or 1/13.

b.      Using Probability

                                                               i.      Genetics uses probability to predict the alleles of the offspring of various crosses.

                                                             ii.      These outcomes are used for comparison with the results of breeding experiments.

                                                            iii.      Genetic ratios are just estimates of probability of an event.

           II.      13.5 Inheritance of Alleles

a.       Monohybrid cross

                                                               i.      Mendel crossed plants that had different pod color.  He crossed true-breeding, green-podded plants with yellow-podded plants.  The plants in this1st cross are called the parental generation (P).

                                                             ii.      Plants that grew from the parental generation are called the first filial (F1) generation.  All were green!

                                                            iii.      F1 generations then self-fertilized, and were called the second filial generation (F2).  428 plants were green-podded, and 152 plants were yellow-podded.  This was a 3:1 ratio.


True Breeding

Parental generation (P)  GG                  X                     gg


meiosis and gamete formation

                   gametes                 G                                             g

                                                                      meiosis and gamete formation


<>                                              F1 Generation                       Gg


                        gametes                        ½ G                 ½ g


                                                G                g      

                                                  GG                Gg     G


                                               Gg                 gg           g


<>                                             F2 Generation                  1GG:2Gg:1gg
                                                                                            3:1 ratio


                                                           iv.      Dominant Traits

1.      The trait that will show up in the F1 generation will show when present.

                                                             v.      Recessive Traits

1.      The trait that doesn't show up in the F1 generation was "covered" by the dominant allele.

                                                           vi.      Principle of Segregation

1.      Gametes form during meiosis and only 1 copy of the factor (green color of pod) is transferred to each pollen or egg cell. 

2.      After fertilization, the F1 generation had a green-podded factor (allele) from 1 parent and a yellow-podded factor from the other parent.  One of these factors is passed into each gamete formed by the F1 plants.

                                                          vii.      Homozygous genotype - Both alleles are the same (GG or gg).
Heterozygous genotype - Alleles are different (Gg).

   Phenotype - The observable characteristics.  [the way the creature looks, genotype + the effects of the environment].

b.      Dihybrid cross

                                                               i.      This is a cross between individuals that differ in 2 traits.

                                                             ii.      Mendel found that the traits of round seeds and yellow embryos are dominant over wrinkled seeds and green embryos.

c.       Mendel’s Principle of Independent Assortment

                                                               i.      Alleles for 1 characteristic divide up between the gametes during meiosis independently of alleles for other characteristics.

Focus On

The Chromosome Theory of Inheritance

Mendel concluded that although egg and sperm cells (or egg/pollen cells) are different and they make equal genetic contributions to the new organism.  Sperm and egg cells are both very similar in structure, but are in different locations.  After many observations of meiosis, it was determined that homologous chromosomes segregate among the gametes like Mendel’s results concluded.  (Mendel did not live to see his pea experiments supported by other research....).

         III.      13.6 Sex Determination

a.       Sex chromosomes in humans are labeled X and Y and are the 23rd pair.  [the other 22 pairs are called autosomal chromosomes]

                                                               i.      Females have 2 X chromosomes, so all haploid eggs produced during meiosis have one X chromosome.

                                                             ii.      Males have 1 X and 1 Y chromosome, so ½ of the sperm have an X chromosome, and the other ½ have a Y chromosome.  The sperm will determine the sex of the offspring of 2 parents.  XX = female, XY = male, in humans.

                                                            iii.      An egg that's fertilized by a sperm with an X chromosome will become a female; an egg fertilized by a sperm with a Y chromosome will become a male.

b.      Sex determination in some insects

                                                               i.      Females have 2 X chromosomes, and males have 1.  There is no Y chromosome.

c.       Sex determination in birds, fish, and some insects (Z-W system)

                                                               i.      Males have 2 matching chromosomes (ZZ), and the female has 1 Z chromosome, and 1 W chromosome.

Other Patterns of Inheritance

              I.      13.7 Multiple Alleles and Alleles without Dominance

a.       Incomplete Dominance

                                                               i.      Ex. Red-flowered snapdragons are crossed with white-flowered snapdragons.

1.      All flowers in the F1 generation have pink flowers which looks like blending, but it's not.  Each allele is equally expressed- equal amounts of  white and red pigment look pink....

                                                             ii.      Ex. Blood Type in Humans

1.      Blood type depends on the presence or absence of type A or B carbohydrates on the surface of red blood cells.  The alleles are labeled IA or IB.

2.      When someone has type IAIB blood, then both carbohydrates are produced and the blood type is AB.  These alleles are called codominant.

3.      Multiple Alleles (more than 2 alleles within the populations gene pool).

a.       i allele type is code for no active enzyme, no A or B types.  An ii genotype produces type O blood.

b.      IA, IB, and i (the 3 human alleles for blood type) are used to show that A and B traits are codominant, and O is recessive.
AA = A
Ai = A
BB = B
Bi = B
ii = O (most common in humans)

4.      When type A blood receives a transfusion of type B blood the red cells will clump together and clog the blood vessels (an immune system response to the foreign B carbohydrate on the surface of the transfused blood).  Antibodies are defensive proteins found in the blood and will bind to foreign substances in the blood.  They are important in defending against infection in the blood stream.

           II.      13.8 Linked Genes

a.       Genes that are on the same chromosome are said to be linked.  They are often inherited together and most of the traits Mendel studied weren't linked genes.

b.      Not all linked genes stay together.  Homologous structures will exchange pieces during their crossing over, which produces a new allele combination.  These greatly increase variation, and thus help EBNS.

c.       The frequency of linked traits becoming separated will reflect on how far apart on the chromosome the genes will be.  This is used to map out the location of genes on chromosomes.

                                      d.       Close together = tightly linked

                     Far apart = loosely linked


         III.      13.9 X-Linked Traits

a.       X-Linked traits are traits whose genes are carried only on the X chromosome. 

b.      In the 1910’s, Thomas Hunt Morgan and his students found a white-eyed male fly.  They mated this fly with a normal red-eyed female fly, and all the offspring ended up with red eyes.  This experiment determined that the white-eye trait is recessive, and more likely to occur in males.

c.       When Morgan’s students found a white-eyed female, they crossed it with a red-eyed male, and the female offspring had only red eyes; the males only white eyes.

d.      These results showed that the gene for eye color is only carried on the X chromosome, not the Y.

        IV.      13.10 Nondisjunction

a.       Nondisjunction is the failure of homologous chromosomes to separate in meiosis.

b.      The recessive eye-color trait is known as vermilion. When vermilion-eyed females were crossed with normal red-eyed males, about once every 2,000 flies a vermilion-eyed female or a red-eyed male appeared in the F1 generation.

c.       The females in the F1 generation must have 2 X chromosomes that are carrying the recessive allele.  These flies must have developed from gametes that were produced by abnormal meiosis.  The offspring of a gamete with 2 sex chromosomes that failed to separate in meiosis, would have an unusual number of sex chromosomes.

d.      Nondisjunction can occur in humans and the effects can be quite severe. 

                                                               i.      Ex. Turner syndrome (XO)  [the "O" meaning no chromosome at all]

1.      Someone with only 1 X chromosome and no Y chromosome are usually short, sexually undeveloped females. 

                                                             ii.      Females with XXX genotype will have limited fertility and may be slightly mentally disabled.

                                                            iii.      Ex. Klinefelter syndrome (XXY)

1.      Males with an XXY chromosome are tall, sexually underdeveloped, and may be slightly mentally disabled.

                                                           iv.      Trisomy-21  (Down syndrome)

1.      An individual receives 3 copies of the 21st chromosome.  This disrupts development and causes limited mental abilities, shortness, characteristic facial features, and heart defects.

2.      Some individuals with Down syndrome are severely retarded, and others only mildly.  About 95% of those with the disease will reach adulthood.  80% of those that reach it will live into their early 50’s.

3.      Down syndrome is the most common serious birth defect in the U.S., and occurs approximately once every 700 births.

           V.      13.11 Multigene Traits (polygenic traits)

a.       Multifactorial traits are affected by several genes and environmental factors.

                                                               i.      Height is considered a quantitative trait (Multifactorial trait).  The environment includes external factors (light and temperature) as well as internal, such as an organism's metabolic activities.

                                                             ii.      Ex. The production of the skin pigment melanin is controlled by 4 major genes which demonstrate simple dominance.  How each of these 4 major gene pairs function determines that persons skin color.

By: Emily Foster, WHS student, 2/24/06

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