• TABLE OF CONTENTS
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 Copyright
 Front Cover
 Introduction
 Genes and chromosomes
 Inheritance of sex
 Notes






Title: 4-H horse program : horse science
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Title: 4-H horse program : horse science
Physical Description: Book
Creator: 4-H Youth Development Program, Florida Cooperative Extension Service, University of Florida
Publisher: 4-H Youth Development Program, Florida Cooperative Extension Service, University of Florida
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Subject: University of Florida.   ( lcsh )
Spatial Coverage: North America -- United States of America -- Florida
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Bibliographic ID: UF00078697
Volume ID: VID00006
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida

Table of Contents
    Copyright
        Copyright
    Front Cover
        Page 1
    Introduction
        Page 2
    Genes and chromosomes
        Page 3 (MULTIPLE)
        Page 4
    Inheritance of sex
        Page 5 (MULTIPLE)
    Notes
        Page 6
        Page 7
        Page 8
Full Text





HISTORIC NOTE


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida





horse science


4-H


HORSE


PROGRAM







NAME


ADDRESS

CLUB


4-H HORSE PROGRAM
HORSE SCIENCE



This educational material has been prepared for 4-H use by the Cooperative Extension Services of the U.S.
Department of Agriculture and State Land-Grant Universities in cooperation with the National 4-H Council and the
American Quarter Horse Association.

Trade or brand names used in the publications are used only for the purpose of educational information. The
information given herein is supplied with the understanding that no discrimination is intended and no endorsement
of products or breeds of horses by the Federal Extension Service or State Cooperative Extension Services is
implied, nor does it imply approval of products or breeds of horses to the exclusion of others which may also be
suitable.

This material was originally published by the National 4-H Council, 7100 Connecticut Avenue, Chevy Chase,
Maryland 20815.

Programs and educational materials of National 4-H Council are available to all persons regardless of race, color,
sex, age, religion, national origin or handicap. Council is an equal opportunity employer.







Horse Science: How Inheritance Works in Horses

Two tiny cells are the only links of inheritance an animal has
with its parents. A sperm cell from the sire and an egg cell from the
dam unite and grow into the new animal.
We know, therefore, that any characteristics inherited from
the parents must come from these two cells. With good care and
good nutrition, the material in the sperm and egg will determine
almost everything about the developing animal its size, its shape,
its color, even is intelligence.
The study of how characteristics are passed from parents to
offspring is the science of genetics. It's easy to see why genetics is
important to horse breeders. In trying to understand the mysteries
of inheritance, geneticists learn things which help to produce better
horses.

GENES AND CHROMOSOMES
Inside the cells of animals are certain complex chemical
compounds. These substances are the carriers of inheritance. They
are called genes and chromosomes.
Chromosomes are long, thread-like structures made of
complex protein. They can be seen with a microscope. In all body
cells except the sperm and the egg, chromosomes exist in pairs.
Each cell contains a certain number of chromosome pairs,
depending upon the animal. Man has 23 pairs of chromosomes in
each of his cells. Here are the number of chromosome pairs for
farm animals.
Horses 33 Pigs 19
Cattle 30 Sheep 27
Goats 30 Chickens 6
Strung along the chromosomes, somewhat like beads on a
string are genes. Genes consist of complex molecules. They are
chemically

NORMAL CELL DIVISION (MITOSIS)


S CHROMOSOME PAIR



DUPLICATE
CHROMOSOME PAIRS


S CELL
[ L_ ( DIVISION


Page 3


linked to the protein of the chromosome. Genes are too small to be
seen with a microscope. But other research methods tell us they are
there.
Genes are the units of inheritance. Characteristics are passed
from parents to offspring through genes. Genes are the "brains" of
the cell. They determine what the cell will be like. This, in turn,
determines what the body will be like.
Since chromosomes come in pairs, so do genes. Two genes
exist side by side, each on one of the chromosomes in the pair. The
total number of genes on a chromosome is not known, but they are
many. And different chromosomes have different numbers of
genes.
The unique thing about genes and chromosomes is that they
are able to reproduce themselves.
As an animal grows, cells divide and form two. Before the
cell divides, each chromosome duplicates itself When the cell
divides one of the duplicates moves into each of the two new cells.
So the two new cells have exactly the same kind and number of
chromosomes. This type of cell division is called mitosis.

CHROMOSOMES IN SEX CELLS
Genes and chromosomes act somewhat differently when
sperm cells and egg cells are formed. In the testes of the male and
in the ovaries of the female, cell division happens another way.
The chromosome pairs separate, one member of each pair
going to one new cell and the other member going to the other new
cell. As these cells divide again, the single chromosomes form
duplicates which go into each of the new cells. This makes the
sperm or the egg contain only a single chromosome of each
original pair of chromosomes. This type of division is called
meiosis.

SEX CELL DIVISION (MEIOSIS)
TESTICLE CHROMOSOME _.L OVARY
CELL PAIR CELL





A H




SUPER EGG CELL
SPERM ,


L ll


DAUGHTER
CELLS


FERTILIZED EGG


FERTILIZATION


(CHROMOSOMES
IN PAIRS AGAIN


June 1989


(I







Horse Science: How Inheritance Works in Horses

In horses, the sperm from the stallion and the egg from the
mare each contain 33 single chromosomes instead of 33 pairs.
Because of the way chromosomes separate at meiosis, millions of
different kinds of sex cells can be produced by one animal.
When fertilization occurs, the single chromosomes from the
sperm join the single chromosomes in the egg. Once again pairs
are formed. So the fertilized egg contains the same number of
chromosome pairs as the cells of the parents.
This fertilized egg develops into a new individual, resembling
each parent in some ways, yet different from them both. And
probably different from any other individual in the world, since the
slightest difference in gene make-up would make a difference in
the animal.

Dominant and Recessive Genes

Most characteristics are determined by several pairs of genes.
For this reason it is impossible to tell exactly what an unborn
animal will look like.
A few characteristics, however, are determined by only one
pair of genes. Black and red coat color in horses is one example.
By studying characteristics such as this, we can learn something
about how inheritance works.
One pair of genes causes the coat to be either black or red,
depending on which particular combination of the two genes is
present. There is one gene for black and a corresponding gene
(allele) for red. The horse will be black if he has two black genes
or if he has one black gene and one red gene. This is because the
black gene is dominant. The horse will be red only if he has two
red genes.
Here's how the genes combine. Let the capital B represent the
black gene. We use the capital because black is dominant. Let the





BLACK
STALLION (BB)
TO RED MARE (bb)


Bb Bb



Bb Bb


ALL FOALS WOULD BE BLACK WITH A
RECESSIVE GENE FOR RED (Bb)


Page 4


small b represent the red gene. Since genes come in pairs, a horse
could have two black genes (BB), one black and one red gene(Bb),
or two red genes (bb). A black horse could have either BB or Bb
genotype. (Genotype means genetic makeup.) A red horse would
have bb genotype. The gene for red (b) is recessive to the dominant
gene for black (B).
Consider this problem: A red (chestnut) mare (bb) is bred to
a truly black stallion (BB). What color will the foal be?
As the genes and chromosomes divide in the mare's ovaries,
the bb genes separate. Each egg contains one b gene. Likewise,
each sperm from the stallion contains one B gene.
When the sperm and egg unite, two genes influencing coat
color are again present. The genotype of the foal will be Bb. Since
the B gene for black dominates the b gene for red, the foal will be
black.
His phenotype (outward appearance) will resemble the
stallion. Both would be black. But their genotypes are different.
The foal is Bb and the stallion is BB.
What then would happen if a black stallion that had a Bb
genotype were bred to a red (bb) mare?
Two possible kinds of sperm would be produced by the Bb
stallion. Half of the sperm would have the B gene and half would
have the b gene.
It would be a 50:50 chance whether the B sperm or the b
sperm united with the b egg from the mare. The genotype of the
foal would be either Bb or bb. Thus half the foals from such a
mating would be black and half would be red.
Suppose a Bb stallion were mated to a Bb mare. Both the
mare and the stallion would be black, but both would carry a
recessive gene (b) for red. Half the sperm would carry the B gene.
Half the sperm would carry the b gene. The same would be true for
the eggs.





BLACK
STALLION (Bb)
TO RED MARE (bb)


Bb bb



Bb bb


HALF THE FOALS WOULD BE RED (bb)
AND HALF WOULD BE BLACK (Bb)


June 1989







Horse Science: How Inheritance Works in Horses

Chances are 25 percent that the foal would have the BB
genotype, 50 percent that it would have the Bb genotype, and 25
percent that it would carry the bb genotype.
Theoretically, of 100 such matings were made, 75 of the foals
would be black. Twenty-five would be red. Of the 75 black foals
only 25 would be truly black (BB) and 50 would carry a recessive
red gene.
What would happen if a red (chestnut) stallion were bred to
a red (chestnut) mare? In this case all the eggs and all the sperm
would carry the b gene. All foals from such matings would be red.
There are also several other pairs of genes that control other
coat colors in horses. The many possible combinations of these
genes cause the many different color patterns we see.

INHERITANCE OF SEX

We can use a similar analysis to show how the sex of a foal
is determined.
In horses, there is one pair of chromosomes which does not
exactly match. One is called the x chromosome and the other, the
y chromosome. Stallions have one x and one y chromosome. Their
sex genotype is xy. Mares have two x chromosomes. Their
genotype is xx. (The small letters x and y do not indicate that either
is dominant or recessive.)
In reduction division in the stallion, half the sperm contain an
x chromosome and half contain a y chromosome. In the mare all
egg cells contain x chromosomes.
If a sperm carrying an x chromosome fertilizes the egg, the
foal will have xx genotype. It would develop as a female. If a
sperm


Page 5


carrying a y chromosome happens to fertilize the egg, the foal
would be xy. It would be a stallion.
The chances are 50:50 for the foal to be male or female.

COMPLICATIONS

So far we have seen how inheritance works in its simplest
form. This basic system forms the pattern for all inheritance.
Complications arise where characteristics are influenced by more
than one pair of genes.
Most of the important traits in horses, such as conformation,
temperament, physical performance, size, muscularity, and
longevity, are influenced by many genes. With 33 pairs of
chromosomes and hundreds of genes involved, it is impossible to
know a horse's complete genotype.
Furthermore, all gene pairs do not work as completely
dominant and recessive. We see this in certain kinds of flowers.
When the red flowering plants pollinate a white flowering plant,
the flowers on the new plant are pink instead or red or white. In
horses, the palomino color pattern is similar to this.
Finally, many things besides the genetic make-up affect a
horse. He may have the genes for running fast, but unless he is fed
properly, well-trained, and protected from injuries he may never
win a race.
A horse with genes for just average temperament that is
properly cared for may have a better disposition than one with
good genes that is treated badly.
Much remains to be learned about inheritance in horses. The
present-day popularity of horses should provide the incentive for
further scientific study in this field.


HOW SEX IS
DETERMINED


MATING OF
BLACK
STALLION (Bb)
TO BLACK MARE (Bb)


BB Bb


lBbl Bb bb

ONE-FOURTH OF THE FOALS WOULD BE
PURE BLACK (BB). ONE FOURTH WOULD
BE RED (bb). HALF WOULD APPEAR
BLACK BUT CARRY THE RECESSIVE
GENE FOR RED (Bb)


ONE-HALF OF THE OFFSPRING WOULD BE
FEMALE AND ONE-HALF WOULD BE MALE


June 1989


XX XY



XX X Y







Horse Science: How Inheritance Works in Horses


NOTES


June 1989


Page 6







Horse Science: How Inheritance Works in Horses


NOTES


June 1989


Page 7



































































1. This document is section 6 of 14 of 4HHSG01, which supersedes CO 201, one of a series of the 4-H Youth
Development Program, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University
of Florida. Date first printed August 1965. Date revised June 1989. Please visit the FAIRS Website at
http://hammock.ifas.ufl.edu.

2. Bobby J. Rankin, New Mexico State University. Debbie Glauer, member of 4-H Animal Science Design Team,
Department of Family, Youth and Community Science, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville, 32611.


-' UNIVERSITY OF
" FLORIDA
Cooperative Extension Service
Instituteof Food and Agricultural Sciences


COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, Christine
Taylor Waddill, Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of
the May 8 and June 30, 1914 Acts of Congress; and is authorized to provide research, educational information and other services only to
individuals and institutions that function without regard to race, color, age, sex, handicap or national origin. The information in this publication
is available in alternate formats. Single copies of extension publications (excluding 4-H and youth publications) are available free to Florida
residents from county extension offices. Information on copies for out-of-state purchase is available from Publications Distribution Center,
University of Florida, PO Box 110011, Gainesville, FL 32611-0011. Information about alternate formats is available from Educational Media and
Services, University of Florida, PO Box 110810, Gainesville, FL 32611-0810. This information was published June 1989 as CO 201, which is
superseded by 4HHSG01, Florida Cooperative Extension Service.




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