Genetics Chapter 11. Basic Terms Trait an inheritable physical characteristic May be internal or...
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Transcript of Genetics Chapter 11. Basic Terms Trait an inheritable physical characteristic May be internal or...
GeneticsChapter 11
Basic Terms Trait – an inheritable physical characteristic
May be internal or external Ex: Eye color, hair color, blood type, personality
Gene - are chemical factors (Segments of DNA) that code for physical traits Allele - Different versions of the genes that may be inherited
2 alleles are inherited for each trait; one from mom and one from dad Some alleles are Dominant and some are Recessive Dominant alleles are represented by a CAPITAL LETTER Recessive alleles are represented by a lowercase letter Ex: Brown hair color is dominant over blonde hair color; if one of each
allele is inherited, only the brown hair color will be seen Phenotype – The physical appearance of the trait Genotype – The genetic description of the trait
Homozygous Dominant – 2 dominant alleles were inherited Homozygous Recessive – 2 recessive alleles were inherited Heterozygous – 1 dominant and 1 recessive allele were inherited
Gregor Mendel Austrian monk who did his work in a monastery
during the mid-1800’s Fertilization is the process of male and female
reproductive cells joining to form a new organism
Mendel became interested in knowing why some pea plants had white flowers (a trait) and some had purple…or, why some pea plants were tall and others short
He fertilized his pea plants and examined the traits of the offspring which later became the basis for modern genetics
More Mendel Mendel started his experiments by looking at
flower color in pea plants Flower color is an example of a Trait
He “self-fertilized” (a form of asexual reproduction) the purple flower plants until ALL of the offspring continuously produced purple flowers. These purple flower plants were True breeding Their genotype was Homozygous Dominant (PP)
Mendel Cont. Next, Mendel self-fertilized plants that had
white flowers, until all the offspring produced had white flowers These offspring were true breeding for white flowers The white flower plants were Homozygous
recessive (pp) He then used a sample of the true breeding
white flowers and the true breeding purple flowers as the “parent generation” in his experiments
The Laws of Genetics Law of Dominance:
The dominant allele will always show in the phenotype unless it is not present.
In other words, the dominant allele masks the presence of a recessive allele
Law of Segregation: Alleles separate during gamete formation (meiosis) This provides each sex cell with one copy of a gene
instead of the usual two Law of Independent Assortment:
Genes are inherited separately from one another due to “crossing-over” during meiosis and chromosome separation during meiosis
True-Breeding Pea Plants
pp = Homozygous Recessive
Cross-Fertilization Mendel took the pollen
from a purple flower plant and used it to fertilize a white flower plant. Predict what the offspring
plants looked like They were all Purple! Their genotypes were
Heterozygous (Pp) These Pp plants were the
F1 Generation
PP pp
All Pp
PG
Cross-Fertilization Again Predict what happened
next when Mendel crossed two Heterozygous Purple plants (Fertilized one plant with
the pollen of the other) 75% of the offspring were
Purple, 25% of the offspring were White These offspring were the
F2 generation
Types of Dominance Some Dominant alleles are not completely
dominant These genes only partially mask the recessive allele if it
is present Therefore, these alleles may show Incomplete
Dominance Ex: A pink flower results from a cross between a red flower
and white flower Some genes may combine 2 dominant alleles
These phenotypes show up together at the same time Therefore, these alleles are Codominant
Ex: A Black and white cow results when a black bull is mated with a white cow
Examples of Incomplete Dom. And Codom.
Incomplete Dominance: Codominance
RR rr
Rr
Exceptions to the Rule Multiple Alleles:
Some genes have multiple alleles EX: Blood type alleles
IA
IB
i Both IA and IB are Dominant alleles whereas “i” is a
recessive allele Not only are there 3 possible alleles (instead of the
usual 2 alleles for most traits), but IA and IB can show codominance
Blood Type Phenotypes There are six possible genotypes of blood
type ii, IAi, IBi, IAIA, IBIB, IAIB
These lead to 4 possible phenotypes A, B, AB or O Side notes:
Type O is the Universal Donor, this means anyone can receive this blood in transfusion, however, Type O patients can ONLY receive Type O blood
Type AB is the Universal Recipient, this means they can receive any blood type in transfusion
Polygenic Traits Some traits are affected
by many genes at the same time These are called Polygenic
Traits Ex: Height, Skin Color, Eye
Color Allows for huge variation in
a poplutation Graphs of these traits for a
given population result in a Bell Curve
Meiosis Meiosis is the process of making gametes The result of meiosis are 4 sex cell with HALF the
normal amount of chromosomes for that organism A normal number of chromosomes = 2N (Diploid)
In humans, there are 23 PAIRS of chromosomes or 46 in total
Each chromosome in a pair has the SAME genes, one came from mom and the other from dad
These chromosomes are HOMOLOGOUS Half the normal amount (found in sex cells) = N
(Haploid) In humans, this would be 23 individual chromosomes
This allows two gametes (sperm and egg) to combine and form a NEW cell with the complete number of chromosomes
Meiosis Continued During Prophase I of
meiosis, Homologous Pairs join together to form Tetrads Remember, one
chromosome from each parent with the same genes
Chromosomes in each tetrad then exchange small segments of DNA This process is called
“Crossing Over” Results in new trait
combination Allows for genes to be
inherited separately from one another (Law of Independent Assortment)
Process of Meiosis Meiosis is broken up into two stages
Meiosis I and Meiosis II During Meiosis one:
Prophase I: Same as mitosis, however, chromosomes form “tetrads”
Metaphase I: Tetrads line up in the middle of the cell (23 pairs line up in the middle)
Anaphase I: Tetrads are ripped apart allowing one chromosome from each homologous pair to move to opposite sides of the cell
Telophase I, Cytokinesis I: Same as mitosis
Process of Meiosis Cont. Meiosis II
THERE IS NO INTERPHASE BETWEEN MEIOSIS I and MEIOSIS II
Prophase II: Same as mitosis prophase Metaphase II: The 23 chromosome “x”s line up in the
middle of the cell Anaphase II: The 23 chromosomes are ripped in half
allowing 23 chromatids to move to each side of the cell Telophase II and Cytokinesis II: Same as usual, however,
at the end, the daughter cells have 23 individual chromosomes
http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=PlayList&p=D2A007AC2BF1878B&playnext=1&playnext_from=PL&index=23