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HUMAN GENETICS AND MUTATIONS
(MODERN GENETICS)
Regents Biology
OBJECTIVES
Upon completion of this unit students will be able to:
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State the chromosome theory.
Explain why the male determines the sex of offspring.
Describe the work that T.H. Morgan and W.S. Sutton did.
Explain why Drosophila flies were ideal for early genetic research.
Describe what a sex-linked trait is and give examples.
Using Punnett Squares predict the probability of having a child with a sex- linked trait when given the
genotype and or phenotype of the parents.
Explain gene linkage and how it is related to crossing-over.
Explain multiple-gene inheritance.
Define mutation.
Differentiate chromosomal and gene mutations.
Understand that for a mutation to be passed on to offspring, it must be present in the gametes (sex cells) of the parent
organism.
List and briefly describe the FOUR types of gene mutations.
Recognize that gene mutations are usually recessive genetic conditions.
Give examples of genetic diseases caused by gene mutations.
Using Punnett Squares predict the probability of having a child with a gene mutation when given
the genotype and or phenotype of the parents.
Identify and describe the SIX examples of chromosomal mutations.
Give examples of genetic diseases caused by chromosomal mutations.
Explain some of the difficulties that arise in studying human genetics.
Briefly describe what genetic counseling is.
Recognize a pedigree chart and identify its purpose.
Describe the processes of amniocentesis and chorionic villus sampling and how they can be used to diagnose
some human genetic disorders.
Construct a karyotype and explain how it can be used to diagnose some human genetic disorders.
Identify the FOUR types of breeding and explain the reasons why breeding is done.
Understand that the environment plays a role in how certain genes are expressed and give an
example.
Recall and understand that each gene carries a separate piece of information that codes for a particular trait
(protein), and relate this to genetic mutations.
KEY WORDS
1.
2.
3.
4.
5.
addition
amniocentesis
chorionic villus
sampling
chromosomal
mutation
deletion
6.
Down
Syndrome
7. gene mutation
8. inversion
9. karyotyping
10. multiple-gene
inheritance
11. mutation
12. sex
chromosomes
13. sex-linked trait
14. sickle-cell
disease
(anemia)
15. translocation
(in genetics)
INTRODUCTION
We have already learned that genes are made up of DNA molecules, which are the simplest
building blocks of heredity. They are grouped together in specific patterns within a person's
chromosomes, forming the unique "blueprint" for every physical and biological characteristic of that
person.
Humans have 46 chromosomes, arranged in pairs in every living cell of our bodies. When the egg
and sperm join at conception, half of each chromosomal pair is inherited from each parent. This
newly formed combination of chromosomes then copies itself again and again during fetal growth
and development, passing identical genetic information to each new cell in the growing fetus.
Current science suggests that human chromosomes carry about 30,000 genes. An error in just one
gene (and in some instances, even the alteration of a single piece of DNA) can sometimes be the
cause for a serious medical condition.
Some diseases, such as Huntington's disease (a degenerative nerve disease) and Marfan
syndrome (a connective tissue disorder), can be inherited from just one parent. Most disorders
cannot occur unless both the mother and father pass along the gene. Some of these are cystic
fibrosis, sickle cell anemia, and Tay-Sachs disease. Other diseases, such as Down syndrome, are
not inherited. In general, they result from an error (mutation) in the cell division process during
conception or fetal development. Still others, such as achondroplasia (the most common form of
dwarfism), may either be inherited or the result of a genetic mutation.
Unlike your parents, you have the option of genetic testing. These tests identify the likelihood of
passing certain genetic diseases or disorders (those caused by a defect in the genes, the tiny,
DNA-containing units of heredity that determine the characteristics and functioning of the entire
body) to your children LIKE 10 YEARS FROM NOW, RIGHT?!?!?!?!. Some of the more familiar
genetic disorders are Down syndrome, cystic fibrosis, sickle cell anemia, and Tay-Sachs disease (a
fatal disease affecting the central nervous system). If your history suggests that genetic testing
would be helpful, you may be referred to a genetic counselor. Or you might decide to seek out
genetic counseling yourself. We’ll talk about the ethics involved in our next unit…..
I. THE CHROMOSOME THEORY
•This theory states that THE ALLELES ARE CARRIED ON
CHROMOSOMES (THE GENES ARE ON THE CHROMOSOMES)
A
L
L
E
CHROMOSOME
L
E
S
•W.S. SUTTON did the initial research on genes and
chromosomes, but the substantial evidence to support Sutton’s
hypothesis was obtained by T.H. MORGAN; Sutton’s hypothesis
led to Morgan’s theory
http://images.google.com/imgres?imgurl=http://www.blackwellpublishing.com/korfgenetics/jpg/300_96dpi/Fig47.jpg&imgrefurl=http://www.blackwellpublishing.com/korfgenetics/figure.asp%3Fchap%3D04%26fig%3DFig47&h=363&w=500&sz=31&hl=en&start=1&tbnid=V0ReDri8IhGS5M:&tbnh=94&tbnw=130&prev=/images%3Fq%3Dalleles%2
Bon%2Bchromosomes%26gbv%3D2%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:enUS:official%26sa%3DG
II. SEXES AND CHROMOSOMES
• HUMANS have 23 pairs of chromosomes in the body (46 total)
• 22 of these are called AUTOSOMES and the 23rd pair contains the SEX
CHROMOSOMES
• The sex chromosomes are the X AND Y chromosomes:
Each egg cell from meiosis receives one X chromosome
Two types of sperm cells are produced from meiosis, X and Y
X
X
XX = female
X
XY = male
Y
•In humans, the SPERM of the male (X or Y) determines the sex of
the offspring
• Example: Do the following cross:
III. T.H. MORGAN
• T.H. Morgan worked with Drosophila (fruit flies) to study
genetics and was very successful
• Why Drosophila?
1. SMALL IN SIZE
2. EASY TO RAISE
3.PRODUCES MANY OFFSPRING
4. LIFE CYCLE IS SHORT AND MANY GENERATIONS CAN BE
STUDIED IN A SHORT TIME
5. ONLY HAVE 8 CHROMOSOMES
http://images.google.com/imgres?imgurl=http://bp0.blogger.com/_lGP8b6RWvYI/RuamajcKaNI/AAAAAAAAAN4/LQddAlS2jb0/s320/Drosophila_melanogaster__side_%252528aka%252529.jpg&imgrefurl=http://orbitalteapot.blogspot.com/2007/09/culturaldrosophila.html&h=248&w=320&sz=13&hl=en&start=34&tbnid=_TpyvEY_N2P_2M:&tbnh=91&tbnw=118&prev=/images%3Fq%3Ddrosophila%26start%3D20%26g
bv%3D2%26ndsp%3D20%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26sa%3DN
IV. SEX-LINKED TRAITS
• The normal eye color for Drosophila is RED
• One day, T.H. Morgan noticed a WHITE-EYED male fly appear in
one of the generations
• What he did:
1.
V. SEX-LINKED TRAITS IN HUMANS
• Defective allele – ABNORMAL ALLELE THAT CAUSES
GENETIC DISEASES
Examples: hemophilia, muscular dystrophy (MD), color and night
blindness
• Color blindness – cannot see RED or GREEN and is more
common in MALES; females can be CARRIERS
Examples using color blindness XB = normal allele
Xb = recessive allele for color blindness
Example 1
VI. GENE LINKAGE
• Every organism has THOUSANDS of genes (gene for eye color, etc.)
• Every organism has a certain number of chromosomes in each BODY
cell (humans have 46)
• Therefore, MANY GENES MUST BE PRESENT ON EACH
CHROMOSOME
• Genes located on the same chromosome are said to be
LINKED because they are inherited together
• If two (or more) genes are linked they DO NOT obey the law of
INDEPENDENT ASSORTMENT
http://scienceblogs.com/gnxp/2007/01/basic_concepts_linkage_disequi.php
VII. CROSSING-OVER
• Crossing over occurs during PROPHASE I of meiosis I
• As a result of crossing-over, the gametes have DIFFERENT gene
linkages than the parent cells
• Genes that are far apart on the same chromosome will become
separated more often than those close together
http://www.phschool.com/science/biology_place/labbench/lab3/images/crossovr.gif
VIII. MULTIPLE GENE INHERITANCE
• Traits that vary in a continuous manner (like height and skin color)
between two extremes ARE NOT controlled by the alleles of a
single gene; they are affected by the alleles of two or more
DIFFERENT genes
IX. MUTATIONS
• Mutation: CHANGE IN DNA
• Factors in the environment that cause mutations are called
MUTAGENS (e.g., asbestos, UV rays)
• Mutations may also be passed on from parent to offspring. For a
mutation to be inherited in a sexually reproducing organism, it
MUST be present IN THE DNA OF THE GAMETE; THE
MUTATION MUST OCCUR IN THE REPRODUCTIVE CELLS
• Inherited mutations are usually RECESSIVE
• Mutations are likely to result in the production of LOW OR NONFUNCTIONING PROTEINS, AND SOME CAN BE FATAL
• Types of mutations:
1. Gene mutation – CHANGE IN THE SEQUENCES OF
BASES ON A GENE
2. Chromosomal mutation – CHANGE IN THE ENTIRE
CHROMOSOME STRUCTURE OR NUMBER
1. GENE MUTATIONS
• Gene mutations are a change in the base sequence of a gene.
In turn, this will affect the PROTEIN that is produced from that
gene.
• Mutation in a SEX CELL (GAMETE) can be passed on to future
generations
• Gene mutations can occur at RANDOM and are typically called
point mutations
DISEASES ASSOCIATED WITH GENE MUTATIONS:
• Phenylketonuria (PKU) – ENZYME THAT BREAKS DOWN
THE AMINO ACID PHENYLALANINE IS ABSENT; RESULTS IN
BRAIN DAMAGE/MENTAL RETARDATION)
•Sickle-Cell Anemia – ABNORMAL RED BLOOD CELLS;
RESULTS IN OXYGEN DEFICIENCY
•Tay-Sachs Disease – ENZYME TO BREAK DOWN LIPIDS IN
THE BRAIN IS ABSENT; RESULTS IN TOO MANY LIPID CELLS
IN THE BRAIN  DEATH
How can you get it?
TYPES OF GENE MUTATIONS
1. ADDITION
• An entire base is ADDED TO the base sequence of a gene
• RESULT: CODONS CHANGE  LOW OR NON-FUNCTIONING
PROTEIN
2. DELETION
• An entire base is MISSING FROM the base sequence of a gene
• RESULT: CODONS CHANGE  LOW OR NON-FUNCTIONING
PROTEIN
3. SUBSTITUTION
• One base is SUBSTITUTED FOR another
• RESULT: CHANGES ONE CODON  LOW OR NONFUNCTIONING PROTEIN
4. INVERSION
• A group of bases are removed and then put back in REVERSE
order
• RESULT: CODONS CHANGE  LOW OR NON-FUNCTIONING
PROTEIN
2. CHROMOSOMAL MUTATIONS
A. Change in chromosome structure
o Usually occurs during meiosis
1. ADDITION – GENES ARE REPEATED ON A
CHROMOSOME (ADDED)
2. DELETION – CHROMOSOME SEGMENT BREAKS OFF
3. TRANSLOCATION – TRANSFER OF A CHROMOSOME
SEGMENT TO A NONHOMOLOGOUS CHROMOSOME
4. INVERSION – A PIECE OF CHROMOSOME IS ROTATED
SO THAT THE GENES ARE REVERSED
B. Change in chromosome number (NONDISJUNCTION)
o Chromosomes that normally separate during meiosis
remain together, which means that AN EXTRA
CHROMOSOME MAY BE PRESENT OR ABSENT
o DISEASES ASSOCIATED WITH NONDISJUNCTION:
1. Down Syndrome – EXTRA CHROMOSOME ON 21ST
PAIR; RESULTS IN MENTAL RETARDATION
2. Turner’s Syndrome – ONLY ONE X CHROMOSOME ON
23RD PAIR; RESULTS IN A SEXUALLY UNDERDEVELOPED
FEMALE
3. Klinefelter’s Syndrome – TWO X’S AND ONE Y (XXY) ON
THE 23RD PAIR; RESULTS IN A MALE THAT APPEARS
NORMAL WITH UNDERDEVELOPED SEX ORGANS
o POLYPLOIDY
• Found in PLANTS
• Cells have a multiple of the normal chromosome number (2n,
5n, 6n, etc.)
• Occurs when CHROMOSOMES FAIL TO SEPARATE
NORMALLY
• Polyploids are usually much LARGER than normal
http://www.bbc.co.uk/scotland/education/bitesize/higher/img/biology/genetics_adaptation/mutations/10polyploid_formation.gif
X. HUMAN GENETIC DISEASES
A. Problems with Studying Human Heredity:
1. LONG LIFE SPAN (HARD TO ANALYZE GENERATIONS)
2. SMALL NUMBER OF OFFSPRING
3. IMPOSSIBLE TO CONTROL EXPERIMENTS
• We use PEDIGREE CHARTS to analyze the inheritance of
genetic traits; these are commonly used to track sex-linked traits
(Color blindness, Muscular Dystrophy, Hemophilia) but are also
used to track autosomal genetic diseases (PKU, Huntington’s,
Tay-Sachs, Cystic Fibrosis)
XI. GENETIC COUNSELING
• Available so that parents can find out if THEY ARE
CARRIERS FOR GENETIC DISEASES
• Amniocentesis – NEEDLE PROCEDURE
PERFORMED ON PREGNANT WOMEN TO TEST
THE DNA OF THE BABY
• Chorionic villus sampling – The chorion of the
placenta is removed for genetic analysis
• Karyotyping – is when a picture of the chromosomes
is taken and compared to a normal human karyotype
to DIAGNOSE AN INDIVIDUAL (OR NOT) WITH A
GENETIC DISEASE
XII. BREEDING
• There are 4 types of breeding:
1. SELECTION – CHOOSING OF ANIMALS AND PLANTS WITH THE
MOST DESIRABLE TRAITS FOR MATING (aka – SELECTIVE
BREEDING)
2. INBREEDING - MATING OF GENTICALLY CLOSE INDIVIDUALS TO
OBTAIN DESIRED RESULTS (BROTHER/SISTER, MOTHER/SON,
FATHER/DAUGHTER, etc.)
• Increases the number of homozygous genes
• May bring out unwanted effects
3. OUTBREEDING – MATING OF INDIVIDUALS NOT CLOSELY
RELATED
• Brings new beneficial alleles
• “Hybrid vigor” = hybrid offspring are superior to the parents (ex. mule)
4. MUTATIONS
• Plant mutations that once occurred and then deemed as useful are
perpetuated by vegetative propagation (ASEXUAL reproduction)
XIII. ENVIRONMENT AND HEREDITY
• The ENVIRONMENT plays a role in how certain
genes are expressed
• Example: the Himalayan rabbit
change in temp.
XIV. THE MOLECULAR BASIS FOR MUTATIONS
• You are such good students that you remember that:
DNA MAKES RNA WHICH MAKES
PROTEINS.
PROTEINS ARE SO VERY IMPORTANT! Our
structures are MOSTLY PROTEINS, enzymes
are PROTEINS (and we know that next to
NOTHING happens without those).
SO, if a mutation is a change in DNA, then
the DNA is all messed up! And If the DNA is
messed up, the PROTEINS that are created
from them will be too!!! Wow-wee!!!