GENETICS NOTES Chapters 12, 13, 14, 15 16
DNA contains the genetic code for the production of PROTEINS. A gene is a segment of DNA, which consists of enough bases to code for many different proteins. The specific proteins produced by a gene determine the TRAITS of an organism.
The traits of that organism can then be passed on to, or INHERITED by their offspring. The study of the inheritance of traits is called GENETICS.
Who is Gregor Mendel? Mendel can be called the FATHER of Genetics because he developed the principles of genetics that we still use today!
Mendel s most significant work was the research and experiments that he conducted by studying PEA PLANTS. These experiments helped him explain how TRAITS were passed from one generation to the next.
Where are these genes? Each gene controls something different, and it has a specific location on a DNA molecule. A DNA molecule that is coiled up is called a CHROMOSOME, which may have more than a thousand genes on it. Your chromosomes are in the NUCLEUS of the cell.
Each species of organism has a different number of chromosomes. Every chromosome carries genes for different traits. Humans have 46 chromosomes in a somatic cell (body cell) and 23 in a gamete (sex cell). In body cells, chromosomes exist in PAIRS.
The pictures below are of a KARYOTYPE. This is a picture of a test that shows human chromosomes that have been cut out and arranged in their pairs.
Who cares about the pairs? Mendel s research showed that there are two factors that control a trait. He concluded that at the end of MEIOSIS, when the gametes are produced, each reproductive cell contains only one factor of each pair.
When the gametes combine during FERTILIZATION, the offspring will have the two factors controlling each specific trait. One chromosome is from the mom and the other chromosome from the dad. Because the pair of factors is separated, or segregated during meiosis, Mendel named this the LAW OF SEGREGATION. Humans have 23 pairs of chromosomes in every cell.
One piece of information that can be found by reading a karyotype is the SEX of the baby. The first 22 pairs of a male and a female are the same; they are called AUTOSOMES or body chromosomes. The 23rd pair isn t really a pair so it is labeled differently. This last pair is considered to be the SEX CHROMOSOMES. These chromosomes determine the sex of the organism. They also carry GENES for other characteristics. The sex chromosome of the female is X chromosome. Most human females have two X chromosomes in every body cell. The sex chromosome only found in males is the Y chromosome. Most human males have one X chromosome and one Y chromosome in every body cell.
What is the sex of person A? How do you know? What is the sex of person B? How do you know?
Are more chromosomes better? It would be easy to think that having an extra chromosome or two would make a superhuman, but this in not the case. Extra chromosomes, damaged chromosomes, and missing chromosomes almost always cause MUTATIONS.
A karyotype can be used to detect some genetic disorders that may occur in a developing baby, or FETUS. Doctors will use cells taken from a developing baby to check that the baby has the correct number of chromosomes. This is done during a procedure called AMNIOCENTESIS.
If there are too many, too few chromosomes, or damaged chromosomes, then that means that there must have been a mistake in MEIOSIS when the GAMETES are formed. If the number of chromosomes is wrong then the TETRADS did not separate correctly during the phase of Meiosis called ANAPHASE I. This mistake in tetrad separation is called NON- DISJUNCTION.
What could happen to the baby? If an egg is fertilized under conditions like these, it is very likely that the mother will miscarry or the baby will die in the first year. If the baby is born, it will have a genetic abnormality. The number of extra chromosomes there are and the pairs effected determine what genetic disorder the baby has and how severe it is.
What is the probability of having a boy or a girl? Many times in genetics, we can figure out how likely it is that a parent will pass something on to their children. To do this we use a tool called a PUNNETT SQUARE. This is a way to show what all of the possible combinations are from the egg and sperm and how LIKELY it is for those combinations to occur
The end product of meiosis produces 1 egg in a woman and 4 sperm in a man. Gametes are haploid, so they have half the number of chromosomes that any body cell has. Gametes only have one of the sex chromosomes that are in any body cell.
REMEMBER the Law of Segregation? A parent randomly passes on only one gene for each trait to each offspring. XY XX You get one chromosome from mom and one from dad randomly. X Y X X
What are the possible sex chromosomes that a woman can give? X or X What are the possible sex chromosomes that a man can give? X or Y
PUNNETT SQUARE The sides of a Punnett Square are for what the gametes could have. In this case, we are investigating sex chromosomes. Write the sex chromosomes that the mother can give on the top and those from the father on the side. Copy the letters from the top of the square into each of the boxes below it. Copy the letters from the sides of the square into each of the boxes next to it. Each of the boxes now shows a possible combination of the egg and sperm. Within all 4 boxes is every possible combination of these two parents.
What are the possible combinations? The baby will either get the combination XX or XY. How many times does each occur? XX 2 out of the 4 times XY 2 out of the 4 times How likely is it for any woman and man to have a boy child? 50% How likely is it for any woman and man to have a girl child? 50% If a woman has a baby boy, what are the chances of her having another baby boy? Why?
How do certain traits get passed on? Non-sex chromosomes are called AUTOSOMES. Every cell of an organism that has been produced by sexual reproduction has two copies of each autosome. The offspring receives one copy of each autosome from each PARENT.
Chromosomes come in pairs you received one from your mother (egg cell) you received the other from your father (sperm cell) Pink = from mom Blue = from dad Chromosome pair 1 pair 2 Chromosome pair 3
These two autosomes, carrying information for the same trait, are called HOMOLOGOUS chromosomes. Homologous chromosomes are the same size and shape and carry genes for the same trait.
Homologous Chromosomes This shows a pair of chromosomes... (chromosome #5 for example)... one from mom and one from dad. Hair color = brown Height = tall Nose size = small Hair color = red Height = short Nose size = big Notice that each chromosome carries genes for the same trait, but the genes can be in different forms.
The different types of genes that are possible for a trait are called ALLELES. Every trait that an organism has is determined by at least two alleles. If a trait is controlled by more than two alleles, it is said to have MULTIPLE ALLELES. Alleles are represented by a letter. Capital letters represent DOMINANT alleles and lower case letters represent RECESSIVE alleles. The trait for a dominant allele will show up over a trait for a RECESSIVE ALLELE.
The picture below is a pair of homologous human autosomes. The genes of an organism make up its GENOTYPE. To represent this, we write the alleles. The genotype of this person (in the figure above) is BB. Both of these alleles are DOMINANT.
If both of the alleles are the same, for example two dominant (GG) or two recessive (gg), the genotype is called HOMOZYGOUS. If the alleles are different (Gg) the genotype is said to be HETEROZYGOUS, and the person who has these genes is a carrier of the recessive gene.
The picture below is a pair of homologous human autosomes. This person s genotype is HOMOZYGOUS DOMINANT. The way that an organism looks is called its PHENOTYPE. If B represents the trait for brown eyes, and b represents the trait for blue eyes, what is the phenotype of this person? This person has BROWN eyes.
List the genotypes: 1. TT 2. Tt 3. tt 4. RR 5. Rr 6. rr List the phenotypes: 1. Tall 2. Tall 3. Short 4. Round 5. Round 6. Wrinkled
List the genotypes: 1. Homozygous Dominant 2. Heterozygous 3. Homozygous Recessive 4. Homozygous Dominant 5. Heterozygous 6. Homozygous Recessive
What are the possible genotypes and phenotypes of the offspring of plants 1 and 3?
POSSIBLE OUTCOMES Possible genotypes: Tt (Heterozygous) Possible phenotypes: Tall All of the offspring will inherit the TALL trait, but each carries the trait for short.
BB Bb bb BB Bb bb B B B b b b B B B b b b B B B B B b B B B B B b b b b b
T t T TT Tt T TT Tt POSSIBLE GENOTYPES: TT- Homozygous Dominant- 2/4-50% Tt- Heterozygous- 2/4-50%
Are all genes either dominant or recessive to determine traits? Certain traits like freckles or dimples are caused by dominant alleles. Only one parent has to have the dominant allele, because there only needs to be ONE dominant gene for the trait to show. Five digits per hand is a recessive trait. In your Punnett square each parent MUST be a carrier or have the trait in order for a child to have 2 recessive genes.
Some traits are neither totally DOMINANT nor totally RECESSIVE. In this case, we cannot tell which gene is dominant over the other. This situation is called INCOMPLETE DOMINANCE. If there is one dominant and one recessive gene, then only the DOMINANT gene is seen. Incomplete dominance will not show either trait. In incomplete dominance, the phenotype of the organism is a mixture of both of the genes. In this case, because both of the traits are dominant, both alleles are written as capital letters.
R R R RR RR 4/4 RR (100% heterozygous) 100% Pink Flowers R RR RR A heterozygous phenotype does NOT show one or the other trait. It shows a combination of the two alleles. R R R RR RR R RR R R ¼ 25% Red ½ 50% Pink ¼ 25% White
B B W BW BW W BW BW All offspring are checkered
When there are two dominant genes, and neither trait is shown because the phenotype is a mixture of the two dominant traits, this is called INCOMPLETE DOMINANCE.
There are other cases, in which both traits show when they are both dominant. This is called CODOMINANCE.
What s the Difference?? COMPLETE dominance: heterozygous individual shows the DOMINANT phenotype (B = brown, b = blue Bb = brown eyes) INCOMPLETE dominance: heterozygous individual shows MIXTURE of both traits (RR = red, R R = white RR = pink)
What s the Difference? CODOMINANCE: heterozygous individual shows BOTH TRAITS EQUALLY (BB = black, WW = white BW = checkered)
Sickle Cell Anemia Sickle-cell anemia is a dangerous, usually fatal genetic disorder. Red blood cells carry OXYGEN through your body. The shape of sickle red blood cells is different from the disklike shape of normal red blood cells. The sickle cells are unable to carry enough oxygen to body parts. Their shape also makes it difficult for them to move through blood vessels, which causes BLOOD CLOTS.
R is the gene for round cell shape and S is the gene for sickle cell shape. FIX YOUR CHART. Put checkmarks in the table to show the shape of cells for persons with the genes listed. SS SR RR What are the phenotypes shown in the chart above? 1. Healthy patient- ROUND red blood cells 2. Healthy patient- ROUND AND SICKLED red blood cells 3. Anemic patient- SICKLED red blood cells What are the genotypes from the chart above? 1. RR 2. SR 3. SS
When genes are codominant, what do you notice about the phenotype of the heterozygous organism? A heterozygous organism SHOWS BOTH TRAITS.
Incomplete Dominance Vs Codominance A YELLOW flower and a BLUE flower are crossed to produce a YELLOW & BLUE SPOTTED FLOWER this is an example of CODOMINANCE GREEN FLOWER- this is an example of INCOMPLETE DOMINANCE Can you explain why?
What about my BLOOD TYPE? When a disease, or ANTIGEN enters your body, your blood cells try to learn the shape of that antigen. Your blood cells will then make ANTIBODIES to fight the antigen in your body. Once your body has destroyed an antigen, it will remember the shape. If you are ever exposed to that antigen again, your blood will remember the antibodies that it made to fight the invader off last time and it will be able to make more.
Your blood type tells doctors what kind of antibodies your blood has. If you were in need of a blood transfusion and doctors put the wrong type of blood in your body, your blood cells would attack the new blood as if it were an ANTIGEN. Some blood types will accept a few different blood types; others will reject all blood types except for a perfect match.
Human blood type genes have multiple alleles that can show regular dominance and recessiveness or codominance. Complete the chart with the possible genes that a person with each of the following blood types could have. REMEMBER: Chromosomes work in pairs, so the person must have TWO alleles to code for a trait. Blood Type Possible alleles A AA or AO B BB or BO O OO AB AB
Blood Type Possible alleles A AA or AO B BB or BO O OO AB AB Which blood types show complete dominance? How do you know? Which blood types show codominance? How do you know?
What possible blood types could a child have if the parents blood types were A and B? For blood type A, the parent could have the genes AA or AO. For blood type B, the parent could have the genes BB or BO. Draw the 4 Punnett Squares to show ALL of the possibilities.
BABY MIX UP Four newborn babies are born in the hospital at the same time. Clumsy Clara the nurse ran in to attach a wristband with each child s name on it, but she dropped the wristbands and they got out of order. Before she alerts the parents, Clara is going to try to figure this out on her own. The only thing she has to help her are the charts with the blood types of all four babies and their parents. Please help her figure out which baby belongs to which family. Luckily, the baby s blood types are all different. Baby A = blood type A Baby B = blood type B Baby C = blood type AB Baby D = blood type O The parents blood types are as follows: The Gonzalez = blood types O and O The Johnsons = blood types O and A The Rogers = blood types AB and B The Smiths = blood types O and AB Write your Punnett Squares on a separate sheet of paper.
The Gonzalez family can only have a baby with blood type O. The Johnsons can have a baby with the blood type A or O. Since Baby D is Baby Gonzalez, the Johnson baby is blood type A. The Rogers can have a baby with blood type A, B or AB. Baby B or Baby C can be theirs. The Smiths can have a baby with the blood type A or B. Baby B is a Smith and Baby C is a Roger.
Are some traits more common in boys than girls? Like autosomes, the sex chromosomes also carry genes for certain traits. The figure below shows some of the traits that are controlled by the GENES on the sex chromosomes. Label the sex chromosomes either male or female. Female Male
How many genes determine whether or not a female has dry skin? How many genes determine whether or not a male has dry skin?
The Y chromosome does not have ALL of the same traits that the X chromosome has. In autosomes, TWO recessive genes must be present for the recessive trait to show. This is different in sex chromosomes.
Sex-Linked Traits Hairy ears in humans: The gene for hairy ears is found only on the Y chromosome (Y-linked). Therefore, males are the only ones to have this sex-linked gene.
Sex-Linked Traits cont. Hemophilia in humans: Babies born with hemophilia are missing or have a low level of a protein needed for normal blood clotting or blood coagulation. The protein is called a clotting factor. Like color-blindness, the gene for hemophilia is recessive and located on the X chromosome (X-linked). This means that males are more likely to be affected than females as explained above.
Sex-Linked Traits cont. Muscular dystrophy: Characterized by progressive weakness and degeneration of the skeletal and voluntary muscles with control movement MD is usually a recessive X-linked disorder, but there are forms that are autosomal.
COLOR BLINDNESS One X-linked is color blindness. Because this trait is on an X- Chromosome, we write X c for recessive or X C for dominant. The dominant trait is normal color vision and the recessive trait is red-green color blindness.
female female male male female Which are red-green colorblind? D and E Which have normal color vision? A, B and C Which has normal color vision even with the gene for color blindness? B In order for a female to have the disease, she must have the genotype X c X c. In order for a male to have this disease, he must have the genotype X c Y.